[Matplotlib-users] draft solution to overlapping dates problem

[John H. <joh...@gm...>]

I don't know if this is helpful or not, but I've seen some complaints  
in the archives about a problem that I've been facing, so I thought  
I'd post my solution.

I am auto-generating graphs of weekly data.  In terms of pixels the  
graphs will always be the same size, but the number of weeks included  
can vary from a single week to two year's worth of data.  For many of  
these graphs matplotlib does a fine job of placing ticks in a helpful  
and readable manner.  But for some situations where I have say, six  
weeks of data, it places ticks every few days in such a way that they  
overlap and are unreadable.

The MaxNLocator is helpful in this situation as I can use it to limit  
the number of ticks, but it isn't ideal because I really only want  
the first day of the week labeled, not some arbitrary location in the  
middle of the weeks.  So I can up with the following locator that  
will place a limited number of labels at places along the axis that  
make sense.  You pass it the max number of bins and the width of the  
data.  So for my needs I give it 6 bins and a width of 7 for weekly  
data.  If there are fewer dates then it only labels the start of each  
week, not days in between.  It there are more weeks than that it  
tries to group them into 6 or fewer bins.  I've done some testing and  
it works well for me.


class MaxNDateLocator(Locator):
     def __init__(self, num_bins, item_width):
         self.bins = num_bins
         self.width = item_width

     def __call__(self):
         self.verify_intervals()
         vmin, vmax = self.dataInterval.get_bounds()
         delta = vmax - vmin
         mul = 1
         if (mul*self.bins*self.width < delta):
             mul = int(delta/(self.bins*self.width)) + 1

         return range(vmin,vmax+1,mul*self.width)