A derisive term coined by Edward R. Tufte to describe any decorative, redundant, or unnecessary visual elements in a chart of graph that do not convey essential information in an accurate and easy-to-understand fashion. Heavy/dark grid lines, 2 1/2D effects, shadows, and gradients and fills, are all considered to be chart junk by purists.
Gradient or hashed fills are a primary example of chartjunk, adding color and lines that obscure data and draw the eye away from the key relationships. Grid lines and tick marks are another, if they dominate the graph rather than supporting it.
Tufte's invention of the sparkline is the ultimate minimalist graph, with no chartjunk: No axes, no grid, no decoration of any kind, just a pure line that invites the user to see only the data trend and nothing else.
Your creative side and/or colleagues may exhort you to "spice it up" or to "add sizzle". Tufte would tell us that our effort is better spent making sure that the data is of high quality and that it stands out:
"The overwhelming fact of data graphics is that they stand or fall on their content, gracefully displayed. ... Forgo chartjunk."
-- Edward R. Tufte, The Visual Display of Quantitative Information
In the extreme case, the chart itself may be chartjunk, if the data will be easier to read and understand in a simple table. It is always worth considering what it is that you want your to visualize, to inform the decision about what chart type (if any) will be used. Once chosen, you should focus on the most effective display of the data and the inferences that you wish to highlight.
1. Edward Tufte forum: Chartjunk
As described in line graph, I am a former creator and purveyor of chartjunk. It is not without irony that I note the appearance of this in New Writeups:
(thing) by Lord Brawl
Girl You Know It's True. When I worked on the charting component, I added 2 1/2D effects, gradient fills, and even (gasp) fills using a provided image (dollar bills or what-have-you). I mostly see the error of my ways now, though I can still be tempted by a snazzy effect. I am not so dogmatic as to believe that the data is all and visual appeal has no place whatsoever.
I find it vaguely amusing that in the push for simplicity, Tufte decided to invent a new word, given that "chart junk" would have worked equally well, without needing to add a new construction to the lexicon.