vwgtimk5 on Log scaling problem

I'm trying to plot data on a log scale using this code fragment:

thingSpeakPlot(time, light, 'YScale','log');

however all I get is an empty chart.  The X-axis has time values, but no Y-axis value or data plotted.  The data is read in correctly - I can see it when I plot it without the log.  Can anyone shed some light on this.