I sometimes get requests for a one- or two-hour abridged version of the two-day data visualization course that I teach. This blog post explains why I always decline such requests. It's fairly challenging to teach people enough to be able “safely” create charts that don’t accidentally mislead or confuse audiences in two full days, and so the main upshot of a one- or two-hour session is likely to be an unwarranted sense of confidence that participants have the necessary skills to reliably create clear, non-deceptive charts.