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About the Chaco Scales package
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In the summer of 2007, I spent a few weeks working through the axis
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ticking and labelling problem. The basic goal was that I wanted to
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create a flexible ticking system that would produce nicely-spaced axis
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labels for arbitrary sets of labels *and* arbitrary intervals. The
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chaco2.scales package is the result of this effort. It is an entirely
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standalone package that does not import from any other Enthought
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package (not even traits!), and the idea was that it could be used in
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other plotting packages as well.
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The overall idea is that you create a ScaleSystem consisting of
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various Scales. When the ScaleSystem is presented with a data range
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(low,high) and a screen space amount, it searches through its list of
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scales for the scale that produces the "nicest" set of labels. It
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takes into account whitespace, the formatted size of labels produced
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by each scale in the ScaleSystem, etc. So, the basic numerical Scales
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defined in scales.py are:
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* FixedScale: Simple scale with a fixed interval; places ticks at
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multiples of the resolution
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* DefaultScale: Scale that tries to place ticks at 1,2,5, and 10 so that
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ticks don't "pop" or suddenly jump when the resolution changes (when
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* LogScale: Dynamic scale that only produces ticks and labels that work
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well when doing logarithmic plots
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By comparison, the default ticking logic in DefaultTickGenerator (in
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ticks.py) is basically just the DefaultScale. (This is currently the
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default tick generator used by PlotAxis.)
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In time_scale.py, I define an additional scale, the TimeScale.
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TimeScale not only handles time-oriented data using units of uniform
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interval (microseconds up to days and weeks), it also handles non-
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uniform calendar units like "day of the month" and "month of the
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year". So, you can tell Chaco to generate ticks on the 1st of every
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month, and it will give you non-uniformly spaced tick and grid lines.
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The scale system mechanism is configurable, so although all of the
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examples use the CalendarScaleSystem, you don't have to use it. In
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fact, if you look at CalendarScaleSystem.__init__, it just initializes
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its list of scales with ``HMSScales + MDYScales``::
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HMSScales = [TimeScale(microseconds=1), TimeScale(milliseconds=1)] + \
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[TimeScale(seconds=dt) for dt in (1, 5, 15, 30)] + \
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[TimeScale(minutes=dt) for dt in (1, 5, 15, 30)] + \
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[TimeScale(hours=dt) for dt in (1, 2, 3, 4, 6, 12, 24)]
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MDYScales = [TimeScale(day_of_month=range(1,31,3)),
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TimeScale(day_of_month=(1,8,15,22)),
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TimeScale(day_of_month=(1,15)),
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TimeScale(month_of_year=range(1,13)),
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TimeScale(month_of_year=range(1,13,3)),
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TimeScale(month_of_year=(1,7)),
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TimeScale(month_of_year=(1,))]
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So, if you wanted to create your own ScaleSystem with days, weeks, and
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whatnot, you could do::
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ExtendedScales = HSMScales + [TimeScale(days=n) for n in (1,7,14,28)]
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MyScaleSystem = CalendarScaleSystem(*ExtendedScales)
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To use the Scales package in your Chaco plots, just import :class:`PlotAxis` from
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:mod:`chaco2.scales_axis` instead of :mod:`chaco2.axis`. You will still need to create a
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:class:`ScalesTickGenerator` and pass it in. The financial_plot_dates.py demo is a
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good example of how to do this.
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docs/source/scales.rst
