This is difficult to reproduce, but it does occur semi consistently.
Allow the data to populate for ~1min. Zoom in on a region of the graph using the mouse select tool. During most tests, the plot is redrawn immediately from right to left with smooth contour. On some instances, the plot is not redrawn, but simply enlarged resulting in a rough look. Screenshots below show the difference.
When zoomed in, it may fetch data from the data buffer depending on what's currently available in the plot cache. It seems that it could be due the high CPU usage when back-filling very dense data from the data buffer.