What is the test for excluding relevant evidence in a trial for a causation defense?

Appellant contends he was denied a fair trial on all the homicide charges because the trial court excluded relevant evidence supportive of his causation defense. "We review a trial court's ruling excluding evidence on grounds of irrelevance [citation] for abuse of discretion. ' " 'The trial court has broad discretion in determining the relevance of evidence [citation] but lacks discretion to admit irrelevant evidence.' " ' " (People v. Thornton (2007) 41 Cal.4th 391, 444.)