Something is wrong with the X-axis labels...
you've got some 30/31's mixed in with 1's...
5/1/06
5/31/06 6/1/06
7/1/06
7/31/06 8/1/06
Unfortunately, there's no way to tell Excel to make the axis ticks "once per month" in the kind of chart I use (called a scatter chart). (I have requested that feature be added.) Excel can only put the ticks evenly spaced. As you know, months are different lengths. Some 30, some 31, and an occasional 28 or 29 days. All you can do is tell Excel to make the ticks every so many days. The normal inclination would be to tell Excel to put them 30 days apart, as that's the number we normally think of as the length of a month, but that works very poorly, because the average length of a month is longer than that. I tell it to put the ticks 30.47 days apart, which gets most of the ticks on the 1st of the month, but there's an occasional tick on the 30th or 2nd of some month. None of this has anything to do with how the data is presented. The data points are all on the 1st or 15th of each month. I've decided to just live with this.
There is another mode in Excel where the X axis becomes nonnumeric and you can label it any way you please. You could label the ticks Joe, Pete, Bob, etc if you wanted. (called a line chart) However, I refuse to use this mode, as in this mode, Excel treats the X axis as nonnumeric, and therefore it becomes impossible to do things such as trend lines, curve fitting, etc. Also in a line chart there isn't any way to deal with nonuniformly space data.
Suppose I decided at some point that grabbing the data twice a month was too much work, and I switched to once a month. I'd want the chart to gracefully handle that, with the points on the right side simply beginning to appear farther apart at some point. A scatter chart treats its input data as (x,y) pairs, so it will put all the dots the right place automatically, and there's no confusion. A line chart treats its data as a sequence of Y values, which it draws evenly spaced, 'cause it knows nothing about any quantitative nature of the X axis. So, unless I'm charting the sales figures for Joe, Pete, and Bob, I always use scatter charts, and because the X axis is often time, I live with the little month size issue. This crazy way we name the days is just a human convention.
