Fix bare epoch time on datetime axes with one data sample

On datetime axes with a single data point, the loop to find the closest distance fails, resulting in a bare epoch time label in the tooltip since closestPointRange is undefined. Setting it to 0 would default it to milliseconds, but we have a little more information than that - if the user has set a minRange, we know the appropriate resolution to use at max zoom. Fallback to 0 anyway, in case they haven't set anything.