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Dash: Layout and interactive

Yu Liang Weng

Yu Liang Weng

12 June 2020 · 11 min read


'Dash is a web application framework that provides pure Python abstraction around HTML, CSS, and JavaScript.'

1. Prerequsites

This documentation assumes some prior knowledge of Python and HTML/CSS. A minimum of being able to install Python, packages, and be able to execute code should get you a example visualisation. If you need assistance with Python, provides documentation and tutorials.

To follow this tutorial, it is best to use JupyterLab (2.0 or above). If you have time, read Dash tutorial and Plot with Python (you'll find some codes are coming from this post). Make sure the following packages are installed:

In addition, datasets we're going to use are Road Safety Data for accidents between 2016-2018 [1] [2] [3] (licensed under Open Government Licence).

2. Setup

Before we can build a Dash app, import all necessary packages and prepare datasets for visualisation.

from jupyter_dash import JupyterDash
import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output
import pandas as pd
import numpy as np

from datetime import datetime as dt
import as px

external_stylesheets = ['']

# create an app
app = dash.Dash(__name__, external_stylesheets=external_stylesheets)

# prepare data
years = [2016, 2017, 2018]
file_list = []
for year in years:
    df = pd.read_csv("data/acc_" + str(year) + ".csv", parse_dates=[9, 11])
    df["Year"] = year

df_acc = pd.concat(file_list)

Of course you can download stylesheets of your choice but make sure it is placed under the assets subdirectory. By doing this you can remove the external_stylesheets parameter in dash.Dash. Do open the stylesheet description and note classes that you can utilise. The directory strcture you should have is:

  • Folder
    • assets
      • style.css
    • data
      • acc_2016.csv
      • acc_2017.csv
      • acc_2018.csv
    • app.ipynb

See completed files from here.

3. Syntax of layout

To create a layout for our app, we should first familarise with Dash's syntax. If you have used ReactJS before, you might have noticed there's a lot of similarities between ReactJS and Dash's layout. Suppose I want to create a title and a subtile in HTML:

    <h1>This is my title</h1>
    <h6>This is my subtitle written in HTML.</h6>

In Dash, these translates to:

    html.H1('This is my title'),
    html.H6('This is my subtitle written in HTML.')

An example on adding styles and classes:

    html.H1('This is my title', style={'marginBottom': '10'}),
    html.H6('This is my subtitle written in HTML.')
  ], className="seven columns", style={'marginTop': '30'})
], className="container")

where html is the dash_html_components we have imported. Note there are few differences:

  • Tag name are capitalised
  • Nested elements are placed inside the children array
  • Elements are seperate by commas
  • The style property is wrap by curly brackets and properties in the style are camelCased
  • className is replacing class
  • You're no longer required to write px after numbers supplied to properties such as margin/padding

For more information on HTML components, visit here.
To run the app, place the following line at the end of all your codes:


4. Create a layout

Now let's create a actual layout for the app. The code block shown below is the base of an app's layout.

app.layout = html.Div(children=[

Then we can add content inside the children array. Use the following code to create a header:

app.layout = html.Div(children=[
        html.H3(children='United Kingdom accidents data'),
        html.H6(children='Accidents overview 2016-2018', style={'marginTop': '-15px', 'marginBottom': '30px'})
    ], style={'textAlign': 'center'})
], style={'padding': '2rem'})

we get the header below(and I have changed the background color in html class in style.css): header

Before you create a layout in Dash, it would be a good idea to have a sketch of what the app would look like, for example draw it on the paper (what I did) or even use powerpoint to design the layout. You could also look at the Dash Gallery for some inspration. I'm using use the following template for my app: fig3

To make the app interactive, Dash core components are vita, actions of app users on these components acts as inputs and we can modify visualisations (outputs) accordingly. We will know more about connecting inputs and outputs in the interactive section, for now we just focus on the layout of inputs and output components. It is your choice of which components to use, of course it will also depends on your data. For datasets in this post, I've chosen variables Date, Day of Week, Accident Severity, and Speed limit as inputs. Inside the layout, I've added the following:

    ################### Filter box ###################### 
        html.Label('Filter by date (M-D-Y):'),
            initial_visible_month=dt(2017, 1, 1),
            min_date_allowed=dt(2016, 1, 1),
            max_date_allowed=dt(2018, 12, 31),
            start_date=dt.strptime("2018-06-01", "%Y-%m-%d").date(),
            end_date=dt.strptime("2018-12-31", "%Y-%m-%d").date()

        html.Label('Day of the week:', style={'paddingTop': '2rem'}),
                {'label': 'Sun', 'value': '1'},
                {'label': 'Mon', 'value': '2'},
                {'label': 'Tue', 'value': '3'},
                {'label': 'Wed', 'value': '4'},
                {'label': 'Thurs', 'value': '5'},
                {'label': 'Fri', 'value': '6'},
                {'label': 'Sat', 'value': '7'}
            value=['1', '2', '3', '4', '5', '6', '7'],

        html.Label('Accident Severity:', style={'paddingTop': '2rem', 'display': 'inline-block'}),
                {'label': 'Fatal', 'value': '1'},
                {'label': 'Serious', 'value': '2'},
                {'label': 'Slight', 'value': '3'}
            value=['1', '2', '3'],

        html.Label('Speed limits (mph):', style={'paddingTop': '2rem'}),
                value=[20, 70],
                    20: '20',
                    30: '30',
                    40: '40',
                    50: '50',
                    60: '60',
                    70: '70'

    ], className="four columns",
    style={'padding':'2rem', 'margin':'1rem', 'boxShadow': '#e3e3e3 4px 4px 2px', 'border-radius': '10px', 'marginTop': '2rem'} )

    ##### HERE insert the code for four boxes & graph #########

As you can see four components are used here. DatePickerRange allows you to select two dates within two preset dates, Dropdown can be use to select day of the week, Checkbox for selecting accident severity, and a RangeSlider for the speed limit range. Depending on your needs and datasets, the general steps of creating a core component could be the following:

  1. Start by decide which part of datasets you want to filter and select a appropriate component from here.

  2. Create the component using dcc.componentName(), then specify id and properties inside the bracket. id is important because you'll need this in the interactive section. Properties of components can be found on the site mentioned in step 1. For example, DatePickRange allows you to specify min_date_allowed and number_of_months_shown, corresponds to the lowest selectable date for the component and the number of calendar months that are shown when calendar is opened. In addition, the multi property allows you to select multiple values.

  3. Double check that the values you have put inside the component is same as in the dataset. As an example, in the options property of the Dropdown component, labels are text displays on the dashboard and the corresponding value is the value we use to filter our datasets, so be sure they are matched.

  4. Style the component and create another one if required. I've added labels just before each component for clarification.

Once done, try run the app with app.run_server(mode="jupyterlab") to review your layout, here's my output:


Next, let's create the layout for outputs. I've inserted the following codes at the end of last code block:

# Number statistics & number of accidents each day

            html.H3(id='no_acc', style={'fontWeight': 'bold'}),
            html.Label('Total accidents', style={'paddingTop': '.3rem'}),
        ], className="three columns number-stat-box"),

            html.H3(id='no_cas', style={'fontWeight': 'bold', 'color': '#f73600'}),
            html.Label('Casualties', style={'paddingTop': '.3rem'}),
        ], className="three columns number-stat-box"),

            html.H3(id='no_veh', style={'fontWeight': 'bold', 'color': '#00aeef'}),
            html.Label('Total vehicles', style={'paddingTop': '.3rem'}),
        ], className="three columns number-stat-box"),

            html.H3(id='no_days', style={'fontWeight': 'bold', 'color': '#a0aec0'}),
            html.Label('Number of days', style={'paddingTop': '.3rem'}),
        ], className="three columns number-stat-box"),
    ], style={'margin':'1rem', 'display': 'flex', 'justify-content': 'space-between', 'width': '100%', 'flex-wrap': 'wrap'}),

    # Line chart for accidents per day
    ], className="twleve columns", style={'padding':'.3rem', 'marginTop':'1rem', 'marginLeft':'1rem', 'boxShadow': '#e3e3e3 4px 4px 2px', 'border-radius': '10px', 'backgroundColor': 'white', }),

], className="eight columns", style={'backgroundColor': '#f2f2f2', 'margin': '1rem'})

Here I've laid out four boxes for text and one box for a line chart, and you might have already noticed that there isn't any contents for components. For the layout of outputs, you only need to specify the type of component you need and give an id for reference (plus any styles you want to add). For example, I want the app to show the number of total accidents, then I use html.H3 to render it and use no_acc as id.
For graphs and charts, you will need to use the dcc.Graph component. This component can be used to render any plotly-powered data visualisation, with dcc.Graph(figure=fig), where fig is a plotly figure. For updating graphs & charts dynamically, dcc.Graph(id='give_me_an_id') is sufficient. There are [other components] you could use for different purposes, for example, upload files, create tabs, and pop-up windows for dialogs. But once you're familar with the graph component then you will find other components follows the same pattern.

5. Interactive

We're only one step to an interactive dash app! In order for inputs and outputs to interact with each other, we use callback to achieve this, this section only introduce to basic callback, if you requires more advance usage, follow the link to learn more. Here is the callback codes for four number statistics:

    [Output(component_id='no_acc', component_property='children'),
     Output('no_cas', 'children'),
     Output('no_veh', 'children'),
     Output('no_days', 'children'),
    [Input('input_date', 'start_date'),
     Input('input_date', 'end_date'),
     Input('input_days', 'value'),
     Input('input_acc_sev', 'value'),
     Input('input_speed_limit', 'value'),
def update_statistics(start_date, end_date, input_days, input_acc_sev, input_speed_limit):
    # filter by date
    df_update = df_acc.loc[(start_date <= df_acc['Date']) & (end_date >= df_acc['Date'])]
    # by weekdays
    if input_days:
        df_update = df_update[df_update['Day_of_Week'].isin(input_days)]
    # by accident severity
        df_update = df_update[df_update['Accident_Severity'].isin(input_acc_sev)]
    # filter by speed limits
    df_update = df_update.loc[(input_speed_limit[0] <= df_update['Speed_limit']) & (input_speed_limit[1] >= df_update['Speed_limit'])]
    days = dt.strptime(end_date, "%Y-%m-%d") - dt.strptime(start_date, "%Y-%m-%d")
    return len(df_update), sum(df_update['Number_of_Casualties']), sum(df_update['Number_of_Vehicles']), days.days

The first part of the callback specify inputs and outputs for your interaction. If you have only one output/input, do not use square bracket around it as this will cause errors. For both outputs and inputs, you'll need to specify component_id and component_property. component_id is the id you have previously given to components, and component_property is the property of the component you would like to pass. For example, Output('no_veh', 'children'), will update the children property of the component with id no_veh; Input('input_date', 'end_date'), will take the end_date property of the component with id input_date as an input.
Immediately after the callback, you'll need to define a function that handles inputs and return outputs. Make sure the number of parameters and the number of outputs matches the number of inputs and outputs in the callback. Orders of parameters and outputs are important too!

Together with the callback for line graphs, you should have the following:


For graph components, there are more works you'll need to do.

######### Callback for accidents line chart ##############################
    Output('acc_line_chart', 'figure'),
    [Input('input_date', 'start_date'),
     Input('input_date', 'end_date'),
def update_line_chart(start_date, end_date):
    # filter by date
    df_update = df_acc.loc[(start_date <= df_acc['Date']) & (end_date >= df_acc['Date'])]
    #acc_by_day = pd.DataFrame(df_update.Date.value_counts().sort_index().reset_index())
    #acc_by_day.columns=["Date", "Number"]
    acc_by_day = df_update.Date.value_counts().sort_index()
    return {
        'data': [dict(
            marker={ 'size': 15, 'opacity': 0.5, 'line': {'width': 0.5, 'color': 'white'} },
            line={'color': "#7bc7ff"}
        'layout': dict(
            title={"text": "Number of accidents occured in the given date range"},
            margin={'l': 40, 'b': 40, 't': 60, 'r': 20},

What you need to return is the same as you would for specify properties for plotly figures. The Python Figure Reference by plotly documents all properties of each graph object can take, this is a very useful documentation for designing your graphs/charts, especially it outlines the type property for each graph object that decides which graph to return:

return {
  'data': [dict(
      type='scatter',      mode='line',

It is likely that you would want to use maps in the future, and specifically, if you want to use mapbox in Dash then an access token is required. Fortunately, you can get it for free by register an account on mapbox and obtain your token on the account page. Here's an example:

return {
    'data': [dict(
    'layout': dict(
        title={"text": "Position of accidents"},
        margin={'l': 20, 'b': 15, 't': 60, 'r': 20},
            "style": "dark", 
            "center": {"lon": -1.474351, "lat": 53.381173},
            "accesstoken": "pk.eyJ1IjoieWx3LXNoZWYiLCJhIjoiY2tiYXpicmNiMGFyYjMwbWJpbGE0Y29odSJ9.ygksJJTy3si1ZGcYb82DpA",
            "zoom": 8,

6. Conclusion

Hopefully this post has given you a gently introduction to layouts and interactive in Dash. It is impossible to include everything in a post but I encourage you to explore documentations that has mentioned above, especially if you requires advanced usage of Dash!



If you wish to deploy the app on web, read this article.

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