I put together a short style guide for chart creation.  I got tired of giving my analysts the same spiel over and over again, so I codified it.  I should have visuals with the style guide, but it would take forever and I think the texts is pretty explanatory.

1. Every chart should have a governing thought or main message.  The title at the top of a page should not be “Monthly Pages Views.”  Rather, there should be a point to why you are showing the user the chart.  A better governing thought would be: “Monthly Pages Views have increased by 12% from the previous month.”
2. Don’t include unnecessary elements in a chart.  Sometimes I see a legend where there is only one data series in the chart. In this case, you would not need the legend; there is only one thing being shown on the chart.  Another example is gridlines.  If knowing the exact numbers of a metric is important to your story, turn on labels and show the numbers.  And borders around charts.  Lets go minimalist in terms of the elements on the chart.
3. No 3d.  It muddies the visual.  See 2.
4. Don’t go over 4 digits on a scale for a chart axis.  There is no room on a page for 7 digit numbers.  One digit is even better.  
5. Clearly label the scale.  If it is not self evident (like months or business units), please clearly label both what the metric is (Page Views, not PVs) and the scale.  If it is in thousands, put that on the axis.  If it is Millions, put that on the axis.  Above all, I am looking for clarity here.  I don’t want people to spend a lot of time figuring out what the “rules of the road” are for a particular chart.
6. Don’t use our internal labels for external consumers.  So, no labeling a chart about page views: “Monthly_pageviews_all.”  Rather “Monthly Pageviews.”  Use plain English, please.
7. Don’t use double axis charts.  I hate them.  If you want to show two different metrics on the same page, just put two charts on the page.  
8. Make sure that the scale for all charts that are using the same metrics are using the same scale.  Changing the scale in the middle of a set of related charts messes with the viewer.  
9. Don’t use line charts for anything that is not time/date based.  Lines imply date or time to a viewer.  
10. Wherever possible, provide some kind of basis for comparison on a chart.  Some options are Year over Year or average.  It is really hard to tell how things are going without a comparison.
11. Don’t vary chart types without good reason.  For example, pie charts and column charts can show the same data.  Viewers get use to seeing a particular type of chart and if you are changing types on them, they have to mentally change gears.  Just pick one type for related charts and stick with it.  And generally, I am not a big fan of Pie’s.  I would prefer waterfall charts, but am not inflexible about it.
12. If you are going to show percentages, then you need to show the total n on the slide.  If someone needs to calculate the counts for the categories on the pie, they need the total n.
13. Must always source the data.  Tell the user where the charts are coming from.  
14. If there are a material number of data points missing, you have to disclose it on the chart as a footnote or include a “missing” category on the chart.  Either way, you need to be explicit about the limits of your analysis.

Any other suggestions?