Following the 10 x 10 post series on tips for better charts, these are the 10 tips for line charts:

  1. Don’t use line markers unless you really need them to identify b&w printed charts;
  2. Don’t use a legend; directly label the series, instead;
  3. If you can’t easily see the pattern of each series you may have too many;
  4. In a time series, the spacing between markers in the x-axis should be proportional. For example, if you have data for years 1980, 1990, 2000 and 2008, the spacing between 2000 and 2008 should be smaller than between other dates; if you can’t do it with line charts use a scatter plot;
  5. If you are comparing two series like imports/exports or profit/expenses, chart the differences, not the actual series (or at least add a small chart with the differences, below the main chart;
  6. If you are comparing two time series with very different units of measurement, consider using a logarithmic scale;
  7. You don’t have to start the Y-axis scale at zero; break the scale if you need;
  8. If you are using different line styles you may be emphasizing some series more than the others; make sure that’s consistent with your users needs (emphasize what is important);
  9. Add a trend line (make sure the trend is plausible…);
  10. Don’t use line charts for categorical data; if you need a profile chart use a scatter plot and switch axis.

Feel free (and I would appreciate) to comment or add your own tips…