Fix log frequency axis labelling sometimes going crazy

The case producing the most ticks, 10-99999 or 11-100000, used to
overflow the maximum of 20 ticks. This ups the maximum to 40 giving,
for the usual case (CD audio) labels at 10,20,50,100,200,500... and
ticks at 30,40,60,70,80. An alternative, of reducing the number of
ticks, leaves 100-1000-10000 as labels and  ticks at 2 and 5 points
between them, which seems less clear.

See github.com/erikd/issues/43

diff --git a/src/spectrogram.c b/src/spectrogram.c
index ae04f53..f8d31a7 100644
--- a/src/spectrogram.c
+++ b/src/spectrogram.c
@@ -251,6 +251,7 @@ y_line (cairo_t * cr, double x, double y, double len)
 ** What's the greatest number of log ticks? 20-22050, the default for CD files,
 ** gives 28 ticks. 20-90000 gives 35.  10-90000 and 20-100000 switch to a
 ** coarser grid with many fewer ticks.
+** Search for "worst case" below for the commentary that says why 35 is the max.
 */
 typedef struct
 {	double value [40] ;	/* More than 35 */
@@ -271,7 +272,7 @@ typedef struct
 */
 
 /* The old code used to make 6 to 14 divisions and number every other tick.
-** What we now mean by "a division" is just the numbered segments so we ask for a
+** What we now mean by "division" is just the numbered segments so we ask for a
 ** minimum of 3 to give the same effect as the old minimum of 6 half-divisions.
 ** This results in the same axis labelling for all maximum values
 ** from 0 to 12000 in steps of 1000 and gives sensible results from 13000 on,
@@ -457,6 +458,7 @@ calculate_log_ticks (double min, double max, double distance, TICKS * ticks)
 	/* Go powering up by 10 from there, numbering as we go. */
 	k = add_log_ticks (min, max, distance, ticks, k, underpinning, true) ;
 
+	// if (log10 (max/min) >= TARGET_DIVISIONS)
 	/* Do we have enough numbers? If so, add numberless ticks at 2 and 5 */
 	if (k >= TARGET_DIVISIONS + 1) /* Number of labels is n.of divisions + 1 */
 	{
@@ -472,6 +474,16 @@ calculate_log_ticks (double min, double max, double distance, TICKS * ticks)
 								underpinning * (1.0 * i), i == 2 || i == 5) ;
 		} ;
 
+	/* Greatest possible number of ticks calculation:
+	** The worst case is when the else clause adds 8 ticks with the maximal
+	** number of divisions, which is when k == TARGET_DIVISIONS, 3,
+	** for example 100, 1000, 10000
+	** the else clause adds another 8 ticks inside each division as well as
+	** up to 8 ticks after the last number (from 20000 to 90000)
+	** and 8 before to the first (from 20 to 90 in the example).
+	** Maximum possible ticks 3+8+8+8+8=35
+	*/
+
 	return k ;
 } /* calculate_log_ticks */