如何在 plotly 中向散点图添加一条线？

Question

Consider the following MWE to draw a scatter plot using the python API to plotly:

import plotly.plotly as py
import plotly.graph_objs
import plotly.offline
plotly.offline.init_notebook_mode()

data = list(range(10))
trace = plotly.graph_objs.Scatter(
    x=list(range(len(data))),
    y=data
)
plotly.offline.iplot([trace])

What if I now want to add a (say) horizontal line to this plot? I went through the documentation, for example the section on line and scatter and that on line charts , but none of the examples seem to cover how to overlay different plots, or simply draw straight lines and similar shapes.

A naive approach to do this is to just add the line as a second scatter plot, like the following:

import plotly.plotly as py
import plotly.graph_objs
import plotly.offline
plotly.offline.init_notebook_mode()

data = list(range(10))
trace = plotly.graph_objs.Scatter(
    x=list(range(len(data))),
    y=data
)
trace_line = plotly.graph_objs.Scatter(
    x=list(range(len(data))),
    y=[4] * len(data),
    mode='lines'
)
plotly.offline.iplot([trace, trace_line])

This approach seems however to be suboptimal: aside for the verbosity required to add a single line, it also makes me manually "sample" the straight line, and it adds the line height to the tooltip on mouse hover.

Is there a better approach to achieve this?

Answer 1

Hi from your question I can see that you need plotly shapes functionality and generate a horizontal line for the plot.

Please find below the code for doing the same graph you have shown in the question

Code:

from plotly.offline import iplot
import plotly.graph_objs as go


data = list(range(10))
trace = go.Scatter(
    x=list(range(len(data))),
    y=data
)
layout = {
    'shapes': [
        # Line Horizontal
        {
            'type': 'line',
            'x0': 0,
            'y0': 4,
            'x1': 10,
            'y1': 4,
            'line': {
                'color': 'rgb(50, 171, 96)',
                'width': 4
            },
        }
    ],
    'showlegend': True
}

fig = {
    'data': [trace],
    'layout': layout,
}


iplot(fig)

Output:

Additional reference:

1. plotly shapes examples

2. plotly shapes reference

Answer 2

Alternatively, you could use the add_shape method, see the doc here. If you add the following code, you could add the line same as y=4 as above.

fig.add_shape(type="line",
              x0=4, 
              y0=0, 
              x1=4, 
              y1=10)

Answer 3

You can just add the next line:

fig.add_hline(y=4, line_width=2, line_dash='dash')

Also checkout the documentation for further deep into the features that plotly recently has added.