Easy Chart "wierdness"

I amusing the AdHoc Trend (that uses the Easy Chart component) and sometimes the pens change from lines:

to what look like high/low bars when I add an additional pen:

Here is a zoomed in portion of the plot:

But, if add another pen it goes back to lines:

I thought it was caused by the amount of data but that does not seem to be the case. Any ideas why the chart is behaving this way?