Plotly: Markers Disappear When (n) Points Are Plotted

Okay, so my initial idea is to make a line plot in plotly and color the line with one color after certain threshold t, and another color before the threshold. It works for a 23 or

Solution 1:

The reason:

This is a 'feature' of Plotly Scatter plots. When (a number) of points are plotted, the underlying Plotly logic converts the 'scatter' plot to a 'line' plot, for rendering efficiency and plot cleanliness. Thus, the markers are converted to a solid line.

The fix:

Simply add mode='lines+markers' to the trace.

Full working example:

This is your source code, with the minor fix mentioned above:

import numpy as np
import plotly.graph_objects as go

X = [j for j inrange(0, 100)]
Y = [j for j inrange(100000, 200000, 1000)]
#X = X[:23]#Y = Y[:23]
X = np.array(X)
Y = np.array(Y)
t = 4

x = X[X <= t]  # Include the threshold
y = Y[X <= t]
bx = X[X >= t]
by = Y[X >= t]

fig = go.Figure()
fig.add_trace(go.Scatter(x=x, y=y, mode='lines+markers', line=dict(width=1, color='grey'), name="useless data"))
fig.add_trace(go.Scatter(x=bx, y=by, mode='lines+markers', line=dict(width=1, color='blue'), name="useful data"))
fig.update_layout(xaxis_title="x axis", yaxis_title="y axis")
fig.show()

Output:

Solution 2:

Bug? Not necessarily. Weird behaviour? Perhaps...

In any case, the solution in your case is:

fig.data[1].mode = 'lines+markers'

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What you seem to be struggling with here is caused by two things:

1. When hovering over a point where there is one trace represented by a line, and one trace represented by a marker, plotly will display the information for the marker even though the line is placed on top.
2. For an increasing length of a go.Scatter() trace, plotly will stop showing markers for after a certain threshold.
3. And this is perhaps the arguably weird part; that the exact threshold does not seem to be determined by the length of the trace alone. We'll take a look at that in the end.

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Details:

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1. Hover behavior

Just run your code as it is, and hover over 4:

Now deselect useless data by clicking the name in the legend and you'll get:

If you zoom in a bit, you'll see that the data is actually there, it just won't show on hover when both traces are activated:

So, what to do about it?

Just include:

fig.data[1].mode = 'lines+markers'

And get:

2. Marker trigger threshold for go.Scatter

In your case, this threshold seems to be a trace with length = 23 since you're seeing the exact behaviour you're describing. So, what's weird about this? The next part:

3. Varying marker trigger threshold

First of all, *why is there a threshold? Probably because a trace with too many markers arguably looks weird:

You found the threshold to be 24. But in a figure built only with go.Figure(go.Scatter(x = x, y = y)), the threshold is 20:

pts = 20x = np.arange(1,pts)
y = np.arange(1,pts)
fig = go.Figure(go.Scatter(x = x, y = y)).show()

pts = 21x = np.arange(1,pts)
y = np.arange(1,pts)
fig = go.Figure(go.Scatter(x = x, y = y)).show()

And I'm not sure why. But I think that would make for a good question on its own.