Clarify the roles of xmin, xmax, ymin, ymax

docs/visual/graph.html

@@ -115,7 +115,7 @@
           <p class="program"> from visual.graph import *		# import graphing features
             <br />
           f1 = gcurve(color=color.cyan)		# a graphics curve<br />
-          for x in arange(0, 8.1, 0.1):		# x goes from 0 to 8<br />
+          for x in arange(0, 8.05, 0.1):		# x goes from 0 to 8<br />
           &nbsp;&nbsp;&nbsp;&nbsp;f1.plot(pos=(x,5*cos(2*x)*exp(-0.2*x)))	# plot          </p>
           <p class="Normal">Importing from <span class="attribute">visual.graph</span> makes available all Visual objects plus the graph plotting module. The graph 
 
@@ -136,7 +136,7 @@
 <p class="Normal"> You can plot more than one thing on the same graph:</p>
 <p class="program"> f1 = gcurve(color=color.cyan) <br />
   f2 = gvbars(delta=0.05, color=color.blue)<br />
-  for x in arange(0., 8.1, 0.1): <br />
+  for x in arange(0., 8.05, 0.1): <br />
   &nbsp;&nbsp;&nbsp;&nbsp;f1.plot(pos=(x,5*cos(2*x)*exp(-0.2*x)))	&nbsp;# curve<br />
   &nbsp;&nbsp;&nbsp;&nbsp;f2.plot(pos=(x,4*cos(0.5*x)*exp(-0.1*x)))# vbars</p>
 <p class="Normal"><strong><font color="#0000A0">Special options for gcurve, gdots, gvbars, and ghbars</font></strong></p>
@@ -186,7 +186,7 @@
 
             and the vertical axis will extend from -200 to +5000. <strong>Currently, xmin and ymin must 
 
-            be negative; xmax and ymax must be positive.</strong></p>
+            be negative; xmax and ymax must be positive.</strong> If you specify xmax but not xmin, it is as though you had also specified xmin to be 0; similarly, if you specify xmin but not xmax, xmax will be 0. The same rule holds for ymax and ymin.</p>
           <p class="Normal">If you simply say <span class="attribute">gdisplay()</span>, the defaults 
 
             are <span class="attribute">x=0</span>, <span class="attribute">y=0</span>, <span class="attribute">width=800</span>, <span class="attribute">height=400</span>, 
@@ -230,7 +230,7 @@
            .....<br />
 
           agelist1<em class="code"> = </em>[5, 37, 12, 21, 8, 63, 52, 75, 7] <br />
-          ages = ghistogram(bins=arange(0, 80, 20), color=color.red) <br />
+          ages = ghistogram(bins=arange(0,80,20), color=color.red) <br />
 		 ages.plot(data=agelist1)	# plot the age distribution<br />
           .....<br />
           ages.plot(data=agelist2)	# plot a different distribution</p>