Meisam,

I'm not certain if this is what is happening in your case, but if the number of points is over the maximum number of markers then some of the less significant points (occluded, close to others, etc) are being hidden. The chart tries to show as many points in diverse areas as possible. You can raise the total number of points shown by increasing maximum markers. But if you raise this too high, you will start to have performance problems from too many markers in the visual tree.

As you zoom in and out, the chart will attempt to keep the same points in view as were visible last time it rendered. As you zoom in, points that were less significant before will become more significant as you zoom in, and so will begin to display.

When you zoom out, however, there will be a high importance on displaying points that were visible the last time the chart was rendered, so there will be a higher weighting on points that you were zoomed in on. This often results in a higher density of points being displayed in one area after you zoom out of it. This is a trade off such that as you are zooming in and out the points don't completely flicker at random as different candidates are selected at each zoom increment, but rather it attempts to select the same points that it did at the last zoom level, if they are still significant and in view.

Does that seem to explain what you are seeing?

-Graham