Formatting dates with scale_x_date in ggplot2

In a previous version of ggplot2, I was able to use one of the two following commands to format my x dates: Either ``` scale_x_date(breaks = "1 month", minor_breaks = "1 week", labels=(date_format="%B")) + ``` or ``` scale_x_date(major="months", minor="weeks", format="%B") + ``` to produce "%B" format, of full month name. (I'm afraid I can no longer distinguish which one worked, because they were both commented out.) I don't recall when, but after updating either R or ggplot in an ubuntu 12.04 upgrade, this no longer worked for me. Now, the very same data produces the error: ``` Error in scale_labels.continuous(scale) : Breaks and labels are different lengths ``` With the first, and ``` Error in continuous_scale(aesthetics, "date", identity, breaks = breaks, : unused argument(s) (major = "months", minor = "weeks", format = "%B") ``` With the second. If I remove the labels= argument, and apply ``` scale_x_date(breaks = "1 month", minor_breaks = "1 week") + ``` it produces a date format of "YYYY-MM-DD" on the first of each month. Consulting with the help for function ?scale_x_date, I've also tried the following: ``` scale_x_date(breaks = "1 month", minor_breaks = "1 week", labels=date_format("%B")) + ``` But this produces this error: ``` Error in structure(list(call = match.call(), aesthetics = aesthetics, : could not find function "date_format" ``` How can I achieve month-name "%B" formatting on my x axis? (If you have any additional insights into the mechanics producing these error messages, I'd also appreciate it.)