Mysterious gaps
 

I am charting a date series on the left column and either 1 or -1 in the right. The right column has either 1 or -1 nothing else. My column chart is set to maximum 1 minimimum -1 so there should be either a coloured bar at the top for 1 values or a black bar down for -1 values (inverted if negative)..

Strangely there are gaps in the top row with no corresponding black down bar and where the gaps are the value is 1 so there should be no gap.

I simply cannot figure out why there are these gaps (spread quite regularly ac4ross the chart).

Any ideas much appreciated, regards, mark