I covered this technique in a recent blog post:Broken Y Axis in an Excel Chart

Basically, the point is that you shouldn't try to represent data in a single chart if you have to resort to breaking an axis. No matter how clearly you try to indicate that the axis scale has a discontinuity in it, the precognitive interpretation that the "broken" values aren't far off from the lower values overrides any conscious effort to make sense of the broken values. Not to mention the approach is a pain to create and maintain.

In the article above, I suggest an alternative approach, which is to make a panel chart, one with a Y scale that includes values up to the break, the other panel with a Y axis scale that includes all values, with the smaller values smushed against the X axis. A panel chart takes some effort, not as much as the broken axis, and if you want, you can simply make two separate charts instead of a si