Make point size and shape configurable

[mddub]

The graph time scale is now configurable. Instead of a fixed 3/4 hours for the 108/144-pixel graph, the point width and margins can be adjusted to show as little as 30 minutes or as much as 12 hours of history. See the new test screenshots for an idea.

• Points can be rectangular or circular
• Points can be separated by a margin and/or joined by a line
• Points can overlap by setting a negative margin, as one way of showing a longer time scale
• Settings page provides a few preset point style options
• Settings page gives feedback on the expected time scale as you adjust the point properties
• Points can be made garishly huge, but this will only be practical when there is an option to make the graph bounds dynamic

As exciting as this is, the real motivation is to make room for predictions, coming next.