James -

IMHO, plotting smoothed curves can distort a chart, and it ranks up
there with 3D effects as techniques I avoid.  If you plot a polynomial
or other fitted line to data, and present it as such, that is not a
distortion.  Showing all the points in addition to the fitted curve can
help illustrate the variability in the data.  Depending on the data and
on the proper use of markers, showing several hundred points on the
chart can improve the information content of the chart.  Small markers
(2-pt squares) or unfilled ones like crosses work pretty well.  Other
ways to indicate variability include judicious use of error bars, or
drawing confidence bands around the fitted curve.

Showing every Nth point only helps to identify different lines in a
chart, helpful if you only have a B&W printer.  I don't think you should
show a marker at every Nth point without having the line connect every
point, to retain a sense of the overall distribution.

Following Edward Tufte (I'm a big fan of his, BTW), you should not use
smoothed curves unless there's an underlying reality to the curve.  Oh,
I said that.  And also, you should probably not eliminate data if the
data adds to the transfer of information about the data (i.e., its
distribution).  Hmm, I said that too, not in so many words.

- Jon
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Jon Peltier, Microsoft Excel MVP
http://www.geocities.com/jonpeltier/Excel/index.html
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