Inspector: rename LineGraphMarker to HorizontalPointMarker. It can be
used to absolutely position, animate, and drag the marker within any
absolute formatting context.
Inspector: add HorizontalRangeMarker and move the "smokescreen" to use this class.
Inspector: fix positioning math for point and range markers.
Inspector: refactor the line graph so the canvas is not the top-level element.
Inspector: fix broken refreshSoon() method of line graph.
Inspector: fix bug where panning a zoomed line graph would result in the graph
being drawn at an inconsistent scale.