Some charts can be hard to read under specific circumstances. For instance, if you have a "Status Over Time" chart which includes both the closed tasks and the open ones, the number of closed tasks will at some point exceed the number of open ones by far. The y-axis range is determined by the largest variable (the closed tasks in this case), which puts the line representing the open tasks very near the bottom of the chart, making it hard to read.

If there was a zoom function, the user could zoom in to the region of interest and get a more accurate look at specific data series.