One test I use to judge the worthiness of data graphics is "self-sufficiency": can the graphical elements stand on their own feet? If one removes the numbers from the graphic, can one still understand the key messages?
Graphical elements such as bars, lines, dots and pie slices encode data into their lengths, widths, heights, sizes, angles and so on. Oftentimes, the actual numbers are printed beside these elements. The numbers may serve one of two purposes: sometimes, to satisfy those readers who would want to know the precise numbers, not just rough visual estimates; sometimes, to cover up flaws in the design because the graphical elements cannot be interpreted without printing the numbers. The "self-sufficiency" test detects this last instance when the graphic designer has failed. In these situations, the data charts are superfluous; the graphical elements did nothing more than regurgitate the underlying tables of numbers.
Below are examples of charts that fail the self-sufficiency test:
(1) The ARV therapy chart
Without the numbers, the Sub-sahara bar becomes rather meaningless: we can't tell how far "off the chart" it is. The message concerning percent of needs met disappears when the right column of numbers is removed.
(2) The Costco chart
When the numbers are removed, there is no scale and thus no ability to gauge the sizes of the dots. The problem of visually estimating dot sizes was addressed here.
The following charts pass the self-sufficiency test:
(1) The poll result bar chart
Here, with numbers removed, the reader can still read off the result with no problems. Of course, it won't satisfy the readers who demand to know precise percentages but then I'd refer such people to an appendix of data tables.
(2) The poll result percentile matrix
The other format used by Clarin in Argentina also preserves its value when the numbers are removed. In this case, patient readers can even read off the precise percentages.
(3) The Bundeslag Bumps chart
All the key messages, concerning changes in party leadership and relative sizes of the parties, are intact. The missing scale is easily remedied by putting one on the side but note that there is no need to print all the data.