[r] Formatting dates on X axis in ggplot2

I'm having a very, very tough time getting the x-axis to look correct for my graphs.

Here is my data (generated via dput()):

df <- structure(list(Month = structure(1:12, .Label = c("2011-07-31", "2011-08-31", "2011-09-30", "2011-10-31", "2011-11-30", "2011-12-31", "2012-01-31", "2012-02-29", "2012-03-31", "2012-04-30", "2012-05-31", "2012-06-30"), class = "factor"), AvgVisits = c(6.98655104580674,7.66045407330464, 7.69761337479304, 7.54387561322994, 7.24483848458728, 6.32001400498928, 6.66794871794872, 7.207780853854, 7.60281201431308, 6.70113837397123, 6.57634103019538, 6.75321935568936)), .Names = c("Month","AvgVisits"), row.names = c(NA, -12L), class = "data.frame")

Here is the chart I am trying to graph:

ggplot(df, aes(x = Month, y = AvgVisits)) + 
  geom_bar() +
  theme_bw() +
  labs(x = "Month", y = "Average Visits per User")

That chart works fine - but, if I want to adjust the formatting of the date, I believe I should add this: scale_x_date(labels = date_format("%m-%Y"))

I'm trying to make it so the date labels are 'MMM-YYYY'

ggplot(df, aes(x = Month, y = AvgVisits)) + 
  geom_bar() +
  theme_bw() +
  labs(x = "Month", y = "Average Visits per User") +
  scale_x_date(labels = date_format("%m-%Y"))

When I plot that, I continue to get this error:

stat_bin: binwidth defaulted to range/30. Use 'binwidth = x' to adjust this.

Despite hours of research on formatting of geom_line and geom_bar, I can't fix it. Can anyone explain what I'm doing wrong?

Edit: As a follow-up thought: Can you use date as a factor, or should you use as.Date on a date column?

To show months as Jan 2017 Feb 2017 etc:

scale_x_date(date_breaks = "1 month", date_labels =  "%b %Y") 

Angle the dates if they take up too much space:

theme(axis.text.x=element_text(angle=60, hjust=1))