double check ranges and don't blithely divide by zero

Reference: /n/sources/patch/plot-range-ck
Date: Wed Jun 19 01:41:28 CES 2013
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/plot/libplot/range.c	Wed Jun 19 01:41:25 2013
+++ /sys/src/cmd/plot/libplot/range.c	Wed Jun 19 01:41:25 2013
@@ -1,9 +1,17 @@
 #include "mplot.h"
 void range(double x0, double y0, double x1, double y1){
+	double d;
+
 	e1->xmin = x0;
 	e1->ymin = y0;
-	e1->scalex = e1->sidex / (x1 - x0 );
-	e1->scaley = e1->sidey / (y1 - y0 );
+	d = x1 - x0;
+	if(d == 0)
+		sysfatal("degenerate x range%g - %g", x1, x0);
+	e1->scalex = e1->sidex / d;
+	d = y1 - y0;
+	if(d == 0)
+		sysfatal("degenerate y range %g - %g", y1, y0);
+	e1->scaley = e1->sidey / d;
 	e1->quantum=e0->quantum/sqrt(e1->scalex*e1->scalex +
 		e1->scaley*e1->scaley);
 	if(e1->quantum < .01)