We have created BTM on page action.
For same we have created incident.
When I try to plot chart in that it shows heat field as failed action when there is browser error.
But there is no Falied Page Action.
Please find attached snapshot .
Have you also created an error rule? If there is no error rule for the Browser error that lets the page action fail you will not see an impact on the page action health.
Could you also please enlarge the marked chart and paint the line in a different color? There might be just a single measurement which is indicated by a yellow point overlapping with the incident field.