Plotting Null Values

I as with many others seem to have a problem with this.

I have a table containing data for four years (monthly) and not all the data starts and finished at the same time period. Using a lookup selectable by user I am able to plot the data for one series against the average. Removing the references prior to the start and after the finish is okay, however where I am stuck is where there are gaps in the series which should not be plotted. If plotting the series manually it is okay as the values are null, but using the lookup feature excel treats the formula itself as a non null value. I would prefer not to use VBA but cannot find any way around this at present.

Any comments would be appreciated

Sorry If I was not too clear

I have attached three jpegs one showing an excerpt from part of the data table, one showing the chart with null values and one without. The data table has around 150 entries and not all entries start and finish at the same time.

The table runs for a four year period and data is displayed only for a 12 month period using a slider and the offset function

The chart shows item 73 which has a gap in the data for Sept and Oct but which shows on the chart as zero values.

If plotted directly from the relevant data there is a gap as it is a null value, but as shown using the lookup function it generates a non null value which excel shows as zero.

Thanks for the tip which I have applied and it works, partly.

The null points are no longer being plotted, however for some reason although the null values leave gaps options has been selected it is interpolating the data (this applies whichever setting is used).

If you have any ideas I would be grateful to hear

I have attached two screenshots illustrating the effect.