Fix Coverity CID 649238 (DIVIDE_BY_ZERO) by validating the reference
column count before using it as a divisor in plotChart().
The reference column count can be cleared when removing reference data,
in on_clearReference_clicked() so ensure chart rendering handles an empty
reference configuration safely.
Signed-off-by: Robin Getz <rgetz503@gmail.com>
Robin Getz
ea0f5fa790