How to make a chart that graphs frequency of entries?

Mike Fitzpatrick's answer will work well, if you're trying to count all values for a month-regardless of year. If you're looking to have a chart of counts for each month by year (i.e. able to differentiate between November 2012 and November 2013), you'll need a little more detail.

1. Create a standard Table from your existing data.
2. Add a column, named Year, with the formula =Year(A2).
3. Add a column, named Month, with the forumla =Month(A2).
4. In your Pivot Table, use Year and Month as your Row Labels, and DateTime as your Values (still with Count).
5. Your Pivot Table will now differentiate the same Month across different years.
6. Finally, with the Pivot Table selected, Insert Column Chart to create a Pivot Chart with your monthly counts.

By using a standard Table for your raw data, as you add values, your Pivot Table will update as will your Pivot Chart.

Assuming your data are valid Excel datetimes, give your range a title (eg. "Datetime") and then create a Pivot Table.

Use Datetime (or whatever you called it) as your Row Labels and your Values. Make sure the Values function is Count, not Sum.

Finally, group your Row Labels by Month and optionally by Year.