Excel should improve dotted-line pattern.
 

In Excel line charts, the dotted and dashed line patterns start over at every
data point. This is different from earlier versions of Excel, in which the
same pattern continued through successive data points. Because of the
problem in the current version, the dots or dashes at each data point may be
close together or even run into each other, which muddles the pattern and the
distinction between different patterns. With the dot-dash pattern, depending
on the spacing between data points, the problem may cause the dots to be
totally obliterated (i.e. they run into the dashes). Would appreciate it if
you could fix this to be like the earier versions of Excel (see above).

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http://www.microsoft.com/office/comm...excel.charting