Enable visualizations to work like the old Chart type where span headings are shown as a separate bit of text below the main X Axis label which then contains the lower-level column heading. For example, if a table has a span "Gender" and the columns below are "M" and "F" then in the visualization show M and F directly on the axis, and below them - centered between them - show "Gender".

Additionally, enable the span heading (ie. "Gender") to be shown on the left (or right) of a bar chart, with "M" and "F" on the opposite side, as a second axis (but not with a second data value, i.e., not a true axis). Conversely, for a column chart, the span heading would appear above the columns, and the column headings below the X Axis.