Change the height of the legend graphic (specifically for fill)

[avsdev-cw]

Currently the height of the "fill" section of the legend "key" (for want of a better word) is fixed:

plotly.js/src/components/legend/style.js

Lines 152 to 153 in 6f34028

	fill.attr('d', pathStart + 'h' + itemWidth + 'v6h-' + itemWidth + 'z')
	.call(fillStyle);

Please could functionality to change this be implemented. A number of our users find the distinction between line and fill keys to be too indistinguishable (particularly when using plots with wider line thicknesses).

I am aware of #3735 but feel this is much more targeted (thus easier) than a general overhaul of the legend styling.