Support time seeking #58

I can use serverDelay as a shift parameter to seek to a region of time. However there are a few issues.

• once you change serverDelay, you still have to wait until the next change event firing for the graph to be refreshed
• The horizon change() method that paints the canvas makes assumptions that dx is always positive (so doesn't copy the correct part of the image in reuse)
• The same method also assumes you are always repainting the "right" side of the chart
• The metric is also assuming it always needs to fetch new values for the "right" side of the graph (rather than past values when seeking back in time).

Time permitting I will try to provide fixes to these issues if I can without completely changing the underlying architectural assumptions.