add grid lines to graphs

[philipp-sumo]

i think it may be helpful to set the "y_extended_ticks" property in metrics graphics.js to "true" for the graphs that are drawn, in order to get a better visual sense of how the daily up/downs in the graphs relate to each other.

[wlach]

Experimented with this, it doesn't really seem to help much for me, but other perspectives welcome:

Here's what it looks like right now, for reference:

[philipp-sumo]

imho having some points of reference is an improvement

[wlach]

@philipp-sumo On reflection I think you are right, I will make this change.