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

[Benjamin R. <ben...@ou...>]

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