plot() is intentionally not provided for twfeCovs() objects (#58):
twfeCovs() estimates one pooled effect per cohort, so there is no
per-(cohort, time) / event-study structure to plot. Use summary() or
tidy.twfeCovs() for the cohort effects. Calling this method always raises
an error.
Usage
# S3 method for class 'twfeCovs'
plot(x, ...)