python - Intelligently calculating chart tick positions -

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whatever i'm using matplotlib, open-flash-charts or other charts frameworks end needing find way set x/y scales limits , intervals since builtins not enough smart (or not @ all...)

just try in pylab (ipyhton -pylab) understand mean:

in [1]: a, b, x = np.zeros(10), np.ones(10), np.arange(10)  in [2]: plot(x, a); plot(x, b)

you'll see , empty frame grid hiding 2 horizontal lines under it's top , bottom borders.

i wonder if there algorithm around (that can port python) set smartly top , bottom y limits , steps , calculate every how many values show x thick.

for example, let's have 475 measures (datetime, temperature) (x, y) with

2011-01-15 10:45:00 < datetime < 2011-01-17 02:20:00

(one every 5 minutes) and

26.5 < temperature < 28.3

my suggestion particular case set:

26.4 <= y_scale <= 28.4 thick every .2

and tick on x_scale every 12 items (once per hour).

but if have 20 measures on 20 days -21.5 < temperature < 38.7, , on? there standardized method around?

the following i've used years simple , works enough. forgive me being c translating python shouldn't difficult.

the following function needed , graphic gems volume 1.

double nicenumber (const double value, const int round) {   int    exponent;   double fraction;   double nicefraction;    exponent = (int) floor(log10(value));   fraction = value/pow(10, (double)exponent);    if (round) {     if (fraction < 1.5)        nicefraction = 1.0;     else if (fraction < 3.0)       nicefraction = 2.0;     else if (fraction < 7.0)       nicefraction = 5.0;     else       nicefraction = 10.0;    }   else {     if (fraction <= 1.0)       nicefraction = 1.0;     else if (fraction <= 2.0)       nicefraction = 2.0;     else if (fraction <= 5.0)       nicefraction = 5.0;     else       nicefraction = 10.0;    }    return nicefraction*pow(10, (double)exponent);  }

use in following example choose "nice" start/end of axis based on number of major ticks wish displayed. if don't care ticks can set constant value (ex: 10).

      //input parameters   double axisstart = 26.5;   double axisend   = 28.3;   double numticks  = 10;    double axiswidth;   double newaxisstart;   double newaxisend;   double nicerange;   double nicetick;      /* check special cases */   axiswidth = axisend - axisstart;   if (axiswidth == 0.0) return (0.0);      /* compute new nice range , ticks */   nicerange = nicenumber(axisend - axisstart, 0);   nicetick = nicenumber(nicerange/(numticks - 1), 1);      /* compute new nice start , end values */   newaxisstart = floor(axisstart/nicetick)*nicetick;   newaxisend = ceil(axisend/nicetick)*nicetick;    axisstart = newaxisstart; //26.4   axisend = newaxisend;     //28.4

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