annotation coordinates do not respect log scaling

When adding annotations to a plot that uses log scaling, the annotation coordinates need to be rescaled to represent the log10 of the actual coordinates. While this is documented behavior, it is rather counterintuitive and should be fixed. Seemingly no other axis-types (date, for instance) needs special treatment but works without modifying the underlying data.

Minimum example demonstrating the point, i.e. label_y is 4 but plotted at 10^4:

df = pd.DataFrame(data={'x':[1,2,3,4],
                        'y':[1,4,9,16],
                        })

fig = px.line(df,
              x='x',
              y='y',
              log_y=True,
              log_x=True,
              )

annotations = []
fig.for_each_trace(
    lambda trace: annotations.append(dict(x=np.log10(np.sqrt(trace.x[0]*trace.x[-1])),
                                          y=np.sqrt(trace.y[0]*trace.y[-1]),
                                          text=f'({np.sqrt(trace.x[0]*trace.x[-1])},{np.sqrt(trace.y[0]*trace.y[-1])})',))
)

fig.update_layout(annotations=annotations,
                  width=300,
                  height=300,
                  )
fig.show()

[nicolaskruchten]

I agree that this is a pretty bad aspect of our API. Specifying layout.(x|y)axis.range on log axes works the same way.

Unfortunately, we will not be able to change this without a major version bump to both Plotly.py and Plotly.js, as it would likely break many people’s code in the wild. The best we can do in the short term is to add a flag to the layout (default False, so folks would have to opt in) that would enable us to accept coordinates directly.

[maegul]

This seems to also wreak some havoc with with figure.add_hline() function and the use of the annotation_position="top left" argument (and similar), as for plotly version 5.4.0 it doesn’t seem to take account of the log axis type.

I found myself needing to make the following adjustment:

(
   fig
    .add_hline(
        y=line_value, 
        annotation_text=f'This is an annotation',
        annotation_position='top left',
        # This is the adjustment --V
        annotation_y=np.log10(line_value)
    )
)