What version of powerpoint? It makes a difference as 2007 uses excel and
previous versions use ms graph... And is it a graph created in powerpoint or
copied/pasted from elsewhere?
Right-click the Y-axis, choose Format Axis. On the Scale tab, put the
appropriate number in the "category X axis crosses at" box.
Or, instead, it might be that you need to change the Tick Mark Labels
setting on the Patterns tab. In that case, right-click the X axis, choose
Format Axis, Patterns tab, and choose Low.