Re: [Matplotlib-users] how to maintain fractional minor tick spacing

On Thu, Apr 12, 2012 at 5:20 PM, Benjamin Root <ben...@ou...> wrote:

>
>
> On Thu, Mar 29, 2012 at 5:53 AM, Christopher Graves <
> chr...@gm...> wrote:
>
>>
>>
>> On Tue, Mar 27, 2012 at 3:31 AM, Mike Kaufman <mc...@gm...> wrote:
>>
>>> On 3/26/12 12:49 PM, Christopher Graves wrote:
>>>
>>>> On Sun, Mar 11, 2012 at 2:32 PM, Christopher Graves
>>>> <chr...@gm... <mailto:chr...@gm...>> wrote:
>>>>
>>>
>>>         Try this:
>>>>
>>>>        from pylab import *
>>>>        from matplotlib.ticker import AutoMinorLocator
>>>>
>>>>        clf()
>>>>        ax=subplot(111)
>>>>        ax.autoscale(tight=True)
>>>>        plot([1,2,4],[1,2,3])
>>>>        ax.xaxis.set_minor_locator(__AutoMinorLocator(2))
>>>>        ax.yaxis.set_minor_locator(__AutoMinorLocator(2))
>>>>
>>>>        draw()
>>>>
>>>>        M
>>>>
>>>>        PS: I believe this is a fairly new feature...
>>>>
>>>>
>>>>    Thanks! Great news that AutoMinorLocator has been added and
>>>>    accomplishes this. Regarding the P.S. I can confirm that the feature
>>>>    was not in matplotlib 1.0.1 - I had to update to 1.1.0 to use it.
>>>>
>>>>    Best /Chris
>>>>
>>>>
>>>>
>>>> Hi Mike,
>>>>
>>>> A follow-up question... When using that, if one then tries to manually
>>>> use the zoom-box tool available with a matplotlib plot, if one draws too
>>>> small of a box (less than 2 major ticks in x or y dimension, based on
>>>> the following error message), it gives the following error and further
>>>> operations on the plot do not work.
>>>>
>>>> ValueError: Need at least two major ticks to find minor tick locations
>>>> ( File "/usr/lib/pymodules/python2.7/matplotlib/ticker.py", line 1528,
>>>> in __call__ )
>>>>
>>>> Any way to avoid this for now? (And ultimately, should this be made into
>>>> a bug fix request?)
>>>>
>>>
>>>
>>> Ok, I seem to remember seeing this error before, but I can't trip it now
>>> (with either 1.1.1rc or today's git checkout of 1.2.x). Do you have
>>> a short script that can reproduce this? For me, the zoom-box tool seems
>>> to be [correctly] setting the majortick locations as I zoom in, thus
>>> preventing this exception. I should note that I'm using the GTKAgg
>>> frontend. This may be the issue. A long time ago I was using the MacOSX
>>> frontend, and maybe this was when I was seeing it...
>>>
>>> Aside from that, this would be a bug.
>>>
>>> M
>>>
>>
>>
>> On Wed, Mar 28, 2012 at 10:50 PM, Christopher Graves <
>> chr...@gm...> wrote:
>>
>>> Hi Mike,
>>>
>>> Ok I found the root cause. Here is a short script:
>>>
>>>
>>> from pylab import *
>>>
>>> from matplotlib.ticker import MultipleLocator, AutoMinorLocator
>>>
>>> plot([0,3],[0,2.2])
>>>
>>> ax = gca()
>>>
>>> ax.xaxis.set_major_locator(MultipleLocator(0.5))
>>>
>>> ax.xaxis.set_minor_locator(AutoMinorLocator(2))
>>>
>>> show()
>>>
>>>
>>> Once MultipleLocator has been called, the auto-reassigning of tick
>>> spacing when zooming (either with the zoom box or the cross and right-click
>>> drag) does not happen, and then AutoMinorLocator has the error because it
>>> has "majorstep = majorlocs[1] - majorlocs[0]" and majorlocs has less than 2
>>> elements when zoomed in that far. (GTKAgg vs others doesn't matter.)
>>>
>>> Seems like a bug. Is it the same in the newer mpl version you have?
>>> For my purposes, a different fix could work, because my reason to use
>>> MultipleLocator is only to make x and y major ticks have equal spacing, as
>>> follows:
>>>
>>> from pylab import *
>>>
>>> from matplotlib.ticker import MultipleLocator, AutoMinorLocator
>>>
>>> ax = subplot(111, aspect='equal')
>>>
>>> plot([0,3],[0,1.1])
>>>
>>> # Set the ticks to have the same interval on both x and y axes:
>>>
>>> x_major_tick_interval =
>>> abs(ax.xaxis.get_ticklocs()[0]-ax.xaxis.get_ticklocs()[1])
>>>
>>> ax.yaxis.set_major_locator(MultipleLocator(x_major_tick_interval))
>>>
>>> # 2 minor ticks per major tick:
>>>
>>> ax.yaxis.set_minor_locator(AutoMinorLocator(2))
>>>
>>> ax.xaxis.set_minor_locator(AutoMinorLocator(2))
>>>
>>> show()
>>>
>>>
>>> aspect='equal' is not necessary to bring out the error, it just
>>> illustrates the purpose of this. Is there another way to fix the x and y
>>> tick interval as equal? (And ideally even maintain the equal spacing when
>>> zooming.. As it is, they initially show as equal, but when zooming they can
>>> lose equal visible spacing while maintaining equal value intervals.)
>>>
>>>
>>> Best,
>>>
>>> Chris
>>>
>>
>>
>> On Thu, Mar 29, 2012 at 4:06 AM, Mike Kaufman <mc...@gm...> wrote:
>>
>>> I can confirm this bug on yesterday's checkout. About equal spacing, I
>>> don't know offhand. A question to ask the list I think. If you could,
>>> please file as an issue on the github tracker. Include your code nugget
>>> that reproduces. Thanks.
>>>
>>> I don't have a lot of time at this moment, so hopefully somebody else
>>> looks at fixing it first.
>>>
>>> M
>>>
>>
>>
>> Ok, bug is filed at https://github.com/matplotlib/matplotlib/issues/807
>> I did not realize that our last couple of messages were not sent to the
>> mailing-list.
>>
>> To others on mailing-list:
>> Apart from someone hopefully fixing this bug, does anyone know another
>> way to fix the x and y tick interval as equal, besides the way I did it in
>> the last code block above, which uses
>> "ax.yaxis.set_major_locator(MultipleLocator(x_major_tick_interval))" after
>> plotting?
>> (And ideally even maintain the equal spacing when zooming.. As it is,
>> they initially show as equal, but when zooming they can lose equal visible
>> spacing while maintaining equal value intervals.)
>>
>> Best /Chris
>>
>>
> Sorry for the long delay in responding.  I have a huge backlog of emails
> to get through.
>
> It sounds like you want ax.set_aspect('equal') or something to that
> effect.  That will maintain it even after zooming.
>
> Ben Root
>
>
No problem! I think I mis-communicated the issue. The example I put above
already does have the aspect ratio 'equal', when preparing the plot: "ax =
subplot(111, aspect='equal')"  The equal-aspect is maintained fine while
zooming. What I also want is to have the x and y tick-spacing be equal and
maintained. Making the tick spacing equal works fine in the way I did it in
the example above, but when you zoom with either of the zoom-tools the
tick-spacing is not maintained equal. (The example above can be run without
the 2 lines near the end that have "AutoMinorLocator", to avoid the actual
bug that occurs when zooming due to AutoMinorLocator.)

Best /Chris