Legend drag gets misaligned in the bottom 2/3 of the plot

[alexcjohnson]

Another one discovered while fiddling re: #1432 - If you make a plot with a legend (perhaps http://localhost:3000/devtools/test_dashboard/#legend_style) and make it editable so you can drag the legend (Plotly.newPlot(gd, gd.data, gd.layout, {editable: true})) you can drag the legend around anywhere in the top 1/3 of the plot area and it positions itself correctly. But if you drag it to the middle or bottom of the plot area, it jumps back up a bit when you drop it.

This comes from a miscalculation with yanchor: auto, which is the default yanchor value, and means that when you're in the middle third of the plot, the position (legend.y) is anchored to the middle of the legend, and when you're in the bottom third it's anchored to the bottom of the legend. This setting is designed to make it easy to keep the legend aligned near a corner of the plot even when the plot size might change.

It seems like x also has this problem, but it has an additional problem that xanchor defaults to left rather than auto as it should. That may be a tougher one to solve, as we can't just change the default without breaking backward compatibility for some plots...

Annotations (when paper-referenced with no arrow) have the same auto anchor behavior, but they do not have either of these bugs.

[etpinard]

cc our new legend expert @antoinerg

[etpinard]

Looks like #4160 made this behave a little bit better:

... but didn't fix it entirely. This bug is essentially what @antoinerg noticed while reviewing in #4160 in #4160 (comment)

[gvwilson]

Hi - this issue has been sitting for a while, so as part of our effort to tidy up our public repositories I'm going to close it. If it's still a concern, we'd be grateful if you could open a new issue (with a short reproducible example if appropriate) so that we can add it to our stack. Cheers - @gvwilson