Cal-HeatMap problem - don't show a calculated value '0.0' or set a min limit?

Dear group members,

sorry I’m not very advanced but really excited about Grafana.

I tried the Cal-HeatMap to view interesting data.
I have two values (missed packets and recovered packets) from a dvb receiver. They increased from time to time but not every loop I push the data with Telegraf. So I used this formula:
missed - recovered = definitely lost.

If there isn’t any new missed and recovered packets at the next loop the value stays at the last level (like a coulter which will be resetted each day at 0:00) will be send to influx.

I uses this metric to do the subtraction:

SELECT non_negative_derivative(mean(“miss_2”)) - non_negative_derivative(mean(“fec_2”)) FROM “exec_custom” WHERE $timeFilter GROUP BY time($__interval) fill(null)

But if there are for example 10 missed packet and 10 recovered packets my result is 0.00 and the result is viewed in the heatmap!? If I hover over the colored fields I got some fields with e.g. ‘0.00 at 2017-04-05 21:00’ (the darkest square in the appended screenshot). But I only want to show values >0 ! Is there any chance to map 0.00 to no data or 0 and not to show this in the heatmap?!
Maybe I do something completely wrong and someone could enlighten me or give me a hint how to do what I want. Maybe my metric is wrong!?

Regards and thanks in advance!


Sorry…solved, replaced mean by max…