Add options to modify the ChartRow vertical axis margin and vertical EventMarker info box offset.

[chrisimmel]

This adds two new props for finer control of vertical spacing:

• EventMarker.infoOffsetY controls the vertical offset in pixels of the EventMarker info box from the top of the chart. The default is 20px, the value that was previously hardcoded. The prop was also added to components that include EventMarker (BarChart, BoxChart, ScatterChart).

• ChartRow.axisMargin controls the vertical margin between the top and bottom of the chart height and the top and bottom of the range of the chart. The default is 5px, the value that was previously hardcoded.

Sample usages were added to the existing stockchart and wind examples.

The motivation for these changes was a LineChart we have at Alluvium that is very squat. The hardcoded values for these parameters took too much vertical space in our use case. In particular, the default offset of the EventMarker info box placed it below the bottom edge of the chart.

[sartaj10]

Closing this. There's another PR for this change