if you use a datetime as x-axis on a bar chart, it is treated as a date and so automatically chooses bins, you can also choose to bin by year/month etc but you can use the actual unique values - i.e. a 'treat as alphanumeric' option.
I'm sorry I can't remember the exact use case. but I was trying to create a bar chart with dates on x axis, but didn't want them linear spaced. e.g. if my data had 1956, 1957, 2011,2012 you might want 4 bars rather than 1 per year. so was suggesting adding the 'treat as alphanumeric' (with a user entered format string like 'yyyy') as is the case for numeric fields.