use middle gray instead of 777 for default parcoords unselected line color

[archmoj]

#6216 exposed parcoords' deselectedLineColor constant which was set to color: 777 which is euqal to 777777.
However it would be best to set the dflt for this unreleased new attribute to 7f7f7f which is our middle gray:

plotly.js/src/components/color/attributes.js

Line 13 in b2df371

'#7f7f7f', // middle gray

@plotly/plotly_js

[alexcjohnson]

That's our default trace color set, not really any statement about what's the "ideal" or "official" gray color. We can adopt it if you think it's better that way, but I wouldn't consider the current color a bug :)

[archmoj]

That's our default trace color set, not really any statement about what's the "ideal" or "official" gray color. We can adopt it if you think it's better that way, but I wouldn't consider the current color a bug :)

The lightness value of 777 (777777) is 0.46 not 0.5 which may not be perfect with some background colors.

[alexcjohnson]

💃