I couldn't set my horizontal axis' label format and it took me a while to figure out why. Looks like you are trying to convert the label format to an int in the DateTimeAxis.CoerceLabelFormat method. Any reason this is in there or is it just leftover from
a copy and paste? I commented out the code below and it works fine.
// Check precision string
if (!int.TryParse(fmt.Substring(1), out precision))