[BUG] Annotations using log axis as reference

[orenbenkiki]

dash (1.1.1)
dash-bootstrap-components (0.7.0)
dash-core-components (1.1.1)
dash-daq (0.1.7)
dash-html-components (1.0.0)
dash-renderer (1.0.0)
dash-table (4.1.0)

Describe the bug

Writing: Scattergl(x_array=[value], ..., layout=dict(..., annotations=[dict(x=value, xref='x', ...)]) will place the annotations directly on top of the marker, as expected.

Writing: Scattergl(x_array=[value], ..., layout=dict(..., xaxis=dict(type='log'), annotations=[dict(x=value, xref='x', ...)]) will not place the annotations directly on top of the marker.

Expected behavior

Adding type='log' to an axis should not require modifying the coordinates of annotations. If an annotation uses this axis as a reference, then the log function should be applied to its coordinate(s), in the same way it is applied to marker coordinate(s).

Workaround

Writing: Scattergl(x_array=[value], ..., layout=dict(..., xaxis=dict(type='log'), annotations=[dict(x=np.log10(value), xref='x', ...)]) will work.

[alexcjohnson]

Unfortunately this is a long-standing plotly.js issue that we can't fix in a backward-compatible way - will have to be a v2.0 change, if & when we get to that. plotly/plotly.js#1258

[orenbenkiki]

There is an easy way to fix it in a backward-compatible way: add a new value for the ref properties that would indicates that the log transformation (and any other axis transformation) will be applied to the annotation coordinate as expected. Say, xref='X' or xref='x!' or xref='x_with_all_transformations'.

This way new code could be written without having to resort to workarounds, while old code would not be impacted. In theory at some point in the future the current 'x' or 'y' value could be deprecated, but that would be far into the future and is completely optional.

There is no reason to wait for a v2.0 version.

[alexcjohnson]

That's an interesting idea - feel free to propose it over at plotly/plotly.js#1258 though, whatever we do it'll happen over there, not in Dash.

I'll note though that the problem is not just with annotations; the axis range has the same issue, as do image positions, and there may be others. Also we need to consider multiple x or y axes (currently xref: 'x2' etc)