Plotly: How to colorcode plotly graph objects bar chart using Python?

Question

def update_graph_bar(named_count,**kwargs):

traces = list()
df = pd.DataFrame(list(Message.objects.all().values()))
available_indicators = list(df['content'].unique())
for t in available_indicators:
    traces.append(go.Bar(
        x=[t],
        y=[df[df['content']==t]['timestamp'].count()],
        name='{}'.format(t),text=[df[df['content']==t]['timestamp'].count()],
        textposition='auto'
        ))
layout = plotly.graph_objs.Layout(barmode='group',paper_bgcolor='#00FFFF',
    plot_bgcolor='rgba(0,0,0,0)',)
return {'data': traces,
      'layout': layout}

I have the above code and here I want to introduce colorcoding using 'marker' in such a way that the color of bargraph should be dependent on its value. as the value increases the color should also change.

Please consider providing a fully runnable code snippet with a sample dataset. As it now stands, people attempting to assist you will spend more time reproducing your problem rather than answering the actual question. — vestland
– vestland, Commented May 19, 2020 at 22:57

vestland · Accepted Answer · 2020-05-20 08:02:36Z

I'm assuming you're looking for something like this:

Plot 1: Plotly express and

Which can easily be produced like this:

import plotly.express as px
data = px.data.gapminder()

data_canada = data[data.country == 'Canada']
fig = px.bar(data_canada, x='year', y='pop',
             hover_data=['lifeExp', 'gdpPercap'], color='lifeExp',
             labels={'pop':'population of Canada'}, height=400)
fig.show()

You can easily adapt that approach to plotly.graph_objects to get:

Plot 2: go.Bar() and 'viridis'

Code 2:

import plotly.graph_objects as go

fig = go.Figure()

x=[1,2,3]
y=[4,5,6]
z=[12,24,48]

fig.add_trace(go.Bar(x=x, y=y,
                     marker=dict(color = z,
                     colorscale='viridis')))

fig.show()

And you can even apply your own custom color scale:

Plot 3: Custom colors

Code 3:

import plotly.graph_objects as go

fig = go.Figure()

x=[1,2,3]
y=[4,5,6]
z=[12,24,48]

customscale=[[0, "rgb(255, 0, 0)"],
            [0.1, "rgb(255, 0, 0)"],
            [0.9, "rgb(0, 0, 255)"],
            [1.0, "rgb(0, 0, 255)"]]

fig.add_trace(go.Bar(x=x, y=y,
                     marker=dict(color = z,
                     colorscale=customscale)))

fig.show()

While code 3 maps colors to relative sizes of a variable, code 4 will show you how you can map colors to absolute values with specified thresholds:

Plot 4: Colors assigned by absolute values of a variable

Code 4:

import plotly.graph_objects as go

fig = go.Figure()

x=[1,2,3]
y=[25,75, 110]
z=[12,24,48]

def SetColor(y):
        if(y >= 100):
            return "red"
        elif(y >= 50):
            return "yellow"
        elif(y >= 0):
            return "green"

fig.add_trace(go.Bar(x=x, y=y,
                     marker=dict(color = list(map(SetColor, y)))))

fig.show()

Is there a way that we can do this but using Plotly Javascript?

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