Line chart is plotting values instead of nulls

I have a line chart used to plot pH readings over time.  I have two sensors.  One sensor stopped reading on a certain date, and another started reading later.  I have a line chart used to plot both.  Even though the first sensor has null values, the line chart shows a straight horizontal line based on the last value available.  How can I remove this line?

The values are null 

Thanks for your help.