[amibroker] Null seems to plot as zero

["Graham Johnson" <grahamj@xxxxxxxxxxxxxxxxxxxxx>]

I'm trying to plot a variable that should only have positive values or 
null

The following line results in the Y axis including zero so all of the 
data is compressed at the top of the chart
Plot(IIf(xaInLongTrade, xaPlotValue, Null), xPlotLabel, colorGreen,
	styleThick);

If the Iif is removed then the result is fine - the Y axis range fits 
the data
Plot(xaPlotValue, xPlotLabel, colorGreen, styleThick);

I've even tried setting any negatives or zeros to null prior to Plot 
but the results are still the same
xaPlotValue = IIf(xaPlotValue <= 0, Null, xaPlotValue);

The battle with this has gone on for over a week now, can anyone help 
please?

Graham


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