add a graph property to disable interactive point selection
interaction model: prevent interactive point selection while panning
change draw point callbacks to take points instead of ...

... individual canvas x and y coordinates
update selection: clear entire interactive canvas when updating selected points

as data points are all redrawn
Dygraph Log10: use Math.Log10 if it's available ...

... it's a native function in modern browsers
allow internal data-point to represent a range of values
find closest point: use point's canvas range values when determining ...


... if coordinates are within a point's value range
always use closest point algorithm on mouse move ...

... instead of closest row (when highlightSeriesOpts not set)