Don't want to plot zero values in line chart

I have dates on an x axis, and two series of data (last year and this year.) Since this year isn't done yet, I want to plot only real values, not zeros (it makes the line crash into the x axis.) Problem is, the data series is a series of formulas calling values from another worksheet, so even an IF statement clause of "" won't work. Excel still plots that value as zero instead of treating it like a blank. Hmm!