Axis auto-range improvements

[etpinard]

This issue attempts to combine ideas from several individual feature requests into coherent picture (i.e. the least number of new attribute possible).

In a private conversation with @alexcjohnson , we proposed adding two new (cartesian) axis attributes: bounds and boundmode. bounds would expect 2-item array values similar to the current range. bounds can be thought of as representing the maximum possible extend of the axis ranges. In turn, boundmode would determine if bounds applies during the auto-range routine (i.e. on first view - discuss in #1861 (comment)) or during interactions as well (as proposed in #887). For example,

xaxis: {
  autorange: true,
  boundmode: 'soft',
  bounds: [0, 100]
}

would restrict the auto-range logic to compute an x-axis range between 0 and 100. This might be useful when plotting percentages where users don't want the axis ranges to go beyond 100%.

xaxis: {
  boundmode: 'hard',
  bounds: [0, 100]
}

would restrict the axis range to [0,100] in the auto-range logic and during interactions.

In doing so, 1-way bounds values such as [null, 100] or [0, null] should be allowed. null items here mean don't restrict the min (or max) bound acting. This behavior is similar to cmin and cmax for color scales. This call signature should also be extended to the axis range attribute - taking care of #400

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A few more things to think about:

• Adding a 'tight' value for axis autorange would take care of Implement different data-based autorange mode #928
• Maybe adding 'once' value for axis autorange for .animate method that can handle any type of figure diff #1849
• Maybe allow span axis range (and bounds) values for Range selectors + streaming and/or markers #387

[alexcjohnson]

One other idea we've discussed is autoranging one axis based on only the data that's visible on the opposite axis or axes - for example zoom in on a particular x range and autorange y based only on the x data in that range. Perhaps autorange: 'visible'?

[petergaultney]

Thanks to @cldougl for pointing me in this direction. I just want to throw my 2 cents in and say that, as per plotly/plotly.py#826, it would be nice if plots where all traces set fill tonexty or tozero did not automatically (or could be configured not to) eliminate all padding on the X and Y axes of cartesian plots. My use case involves adding markers to the lines that make up the filled traces, and those markers get "cut off" near the edges by the forced elimination of all padding around the plot.

[alexcjohnson]

@petergaultney thanks for the note - we definitely intend to make all of this more configurable. But another (fairly new) option that might be useful to you is plotly/plotly.py#1861 - cliponaxis: false, perhaps in conjunction with the layout option margin.pad.

[petergaultney]

sure enough, with those two options combined my problem is actually solvable. I set cliponaxis=False on each of my traces, and then added margin.pad on the layout. thanks!!

[etpinard]

There's also #1022 about adding rangemode="symmetric" which sounds very reasonable.

[alexcjohnson]

From plotly/plotly.py#2083 - certain trace types (bars, fill to zero) currently force the axis autorange to include zero. We should let the user override that.