When creating a DataChart using area or dashed line, it is common to use hair (area) or xsmall (line) thickness. But, this affects the display of the legend. Hair-width does not allow for distinguishing between colors. For lines, an additional issue is the use of a dotted line. The color swatch for a legend should automatically (or explicitly, identically) set thickness for all legend items and should only pick up on opacity and dash from the per-series settings.

In the first example below, the areas are set to hair thickness and the line is dashed and xsmall. The resulting legend is terrible.
In the second example, the thicknesses are all undefined. This results in an unusable graph, but a reasonable legend, although dash should be considered in a legend.