Minor UX cleanups for performance dashboard

From the latest mockup, shown in https://gitlab.com/gitlab-org/gitlab-ce/issues/30369, there are various discrepancies. Here are some of them:

X-axis: missing ticks
X-axis: the space around the label should be the same as the Y-axis label
Y-axis: horizontal lines behind the chart should be lighter colored
Both axis: the text should have a lighter gray color
Legend: increase the space between it and the chart
Legend: reduce size of the color block
Tooltip: increase padding (and maybe reduce the font size — I'm not sure about this because I'm not aware of the scale of the screenshot)

Edited Jul 04, 2017 by silv