Axis ticks color

[jjagielka]

There's a common D3 axis trick to set the ticks hight to a negative value to obtain a grid.
We can use that trick here as well, but the lines are drawn with stroke "#000" resulting in strong black lines.

A quick improvement would be to change the stroke value to "currentColor".

Ref: https://css-tricks.com/currentcolor/

[ffigiel]

It's possible to override the color with css

    .chart_axisGrid .tick > line {
      stroke: $grey-lighter;
    }

[gampleman]

I guess the idea here is that currentColor might be a better default, since the charts would adapt automatically to the colour of surrounding typography. However, this would be a breaking change (as the output of axes is to be depended upon), so I am unsure about this. I'll punt on it until we are ready to make the next major release (which to be clear, is not going to be in the short term).