Path: utzoo!utgpu!watmath!clyde!att!rutgers!mailrus!ncar!ames!pasteur!miro.Berkeley.EDU!ph
From: ph@miro.Berkeley.EDU (Paul Heckbert)
Newsgroups: comp.graphics
Subject: Re: I need a wheel...
Message-ID: <7945@pasteur.Berkeley.EDU>
Date: 2 Dec 88 22:34:51 GMT
References: <3343@ucdavis.ucdavis.edu>
Sender: news@pasteur.Berkeley.EDU
Reply-To: ph@miro.Berkeley.EDU (Paul Heckbert)
Organization: University of California at Berkeley
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In article <3343@ucdavis.ucdavis.edu> vmrad@deneb.ucdavis.edu
asked for code to label axes with pleasing numbers and place tick marks
at pleasing intervals.

Here is some code I've used.
The heart of it is a simple little routine called "nicenum" that picks
"nice-looking" numbers.

The below is geared toward y axis labeling but of course it could
work equally well for the x axis.

Paul Heckbert, CS grad student
508-7 Evans Hall, UC Berkeley		UUCP: ucbvax!miro.berkeley.edu!ph
Berkeley, CA 94720			ARPA: ph@miro.berkeley.edu

/*
 * label: test program to demonstrate nice graph axis labeling
 *
 * Paul Heckbert, 2 Dec 88
 */

#include <stdio.h>
#include <math.h>
double expt(), tick(), nicenum();

#define NTICK 5			/* desired number of tick marks */

main(ac, av)
int ac;
char **av;
{
    double ymin, ymax;

    if (ac!=3) {
	fprintf(stderr, "Usage: label <ymin> <ymax>\n");
	exit(1);
    }
    ymin = atof(av[1]);
    ymax = atof(av[2]);

    ylabel(ymin, ymax);
}

ylabel(ymin, ymax)
double ymin, ymax;
{
    char str[6], temp[20];
    int exp;
    double graphymin, graphymax, range, d, y;

    /* we expect ymin!=ymax */
    range = nicenum(ymax-ymin, 0);
    d = nicenum(range/(NTICK-1), 1);		/* tick mark spacing */
    graphymin = floor(ymin/d)*d;
    graphymax = ceil(ymax/d)*d;
    exp = floor(log10(d));
    sprintf(str, "%%.%df", exp<0 ? -exp : 0);	/* simplest axis labels */

    printf("graphymin=%g graphymax=%g increment=%g\n", graphymin, graphymax, d);
    for (y=graphymin; y<graphymax+.5*d; y+=d) {
	sprintf(temp, str, y);
	printf("(%s)\n", temp);
    }
}

/*
 * nicenum: find a "nice" number approximately equal to x
 * round if round=1, ceil if round=0
 */

static double nicenum(x, round)
double x;
int round;
{
    int exp;
    double f, y;

    exp = floor(log10(x));
    f = x/expt(10., exp);	/* fraction between 1 and 10 */
    if (round)
	if (f<1.5) y = 1.;
	else if (f<3.) y = 2.;
	else if (f<7.) y = 5.;
	else y = 10.;
    else
	if (f<=1.) y = 1.;
	else if (f<=2.) y = 2.;
	else if (f<=5.) y = 5.;
	else y = 10.;
    return y*expt(10., exp);
}

/*
 * expt(a,n)=a^n for integer n
 * roundoff errors in pow were causing problems, so I wrote my own
 */

double expt(a, n)
double a;
register int n;
{
    double x;

    x = 1.;
    if (n>0) for (; n>0; n--) x *= a;
    else for (; n<0; n++) x /= a;
    return x;
}