A broken axis graph is one in which part of the scale on the x or y axis has been omitted to save space. The broken axis graph has a wavy line at the location where the scale is broken.
Broken scale graphs are useful in situations when the numbers near zero are not needed. For example, if one of the axes measures temperature during the summer months, it is unlikely that temperatures are below 20 degrees Celsius. In this case, breaking the axes serves to "zoom in" on the important data. Broken axes should only be used to make existing relationships more apparent. Broken axes should not be used to distort the nature of the data.