[FIXED JENKINS-22686] Add a few pixels distance between tick labels.

Because *this* isn't that easy to read:

![Screen shot](https://cloud.githubusercontent.com/assets/1831569/2749434/b34540fe-c801-11e3-9f46-121614111385.png)

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### PR notes

`6.0` is rather arbitrary, but seems to work well. Labels text is ~10 px size, so this results in between half and one text height of distance. Could be made configurable using system property, but I don't see the need. Can be added later at any time.

While for the `MultiStageTimeSeries`, only right margin would be needed, I added all four, as the rest of the code also supports other orientations. The left margin is useless (except for consistency) though, could leave that out if it bothers someone.