PSYM=3 (single-pixel dot) is not a particularly good symbol for ***sparse*** plots. It can be very hard to see the contrast between a single illuminated pixel and its background. This example illustrates: IDL> device, DECOMPOSED=1 IDL> plot, findgen(10), PSYM=3, BACKGROUND=0, COLOR='0000FF'X ; red on black What PSYM=3 is good for is dense scatter plots, the kind of plotting that might be done with PLOTS, for example. That is, PSYM=3 shows a "cloud of data" better than any other symbol. James Jones