add as_heatmap to "image" doc

README.pod

@@ -528,11 +528,41 @@ as they are interpreted as 8 bits per plane colour values. E.g.:
   $image_data = rpic( 'my-image.png' )->mv(0,-1); # need RGB 3-dim last
   $w->image( $image_data );
 
+If you have a 2-D field of values that you would like to see with a heatmap:
+
+  use PDL::Graphics::ColorSpace;
+  sub as_heatmap {
+    my ($d) = @_;
+    my $max = $d->max;
+    die "as_heatmap: can't work if max == 0" if $max == 0;
+    $d /= $max; # negative OK
+    my $hue   = (1 - $d)*240;
+    $d = cat($hue, pdl(1), pdl(1));
+    (hsv_to_rgb($d->mv(-1,0)) * 255)->byte->mv(0,-1);
+  }
+  $w->image( as_heatmap(rvals 300,300) );
+
 =item contours
 
 As of 1.012. Draws contours. Takes a 2-D array of values, as (width x
 height), and optionally a 1-D vector of contour values.
 
+=item fits
+
+As of 1.012. Displays an image from an ndarray with a FITS header.
+Uses C<CUNIT[12]> etc to make X & Y axes including labels.
+
+=item polylines
+
+As of 1.012. Draws polylines, with 2 arguments (C<$xy>, C<$pen>).
+The "pen" has value 0 for the last point in that polyline.
+
+  use PDL::Transform::Cartography;
+  use PDL::Graphics::Simple qw(pgswin);
+  $coast = earth_coast()->glue( 1, scalar graticule(15,1) );
+  $w = pgswin();
+  $w->plot(with => 'polylines', $coast->clean_lines);
+
 =item labels
 
 This places text annotations on the plot.  It requires three input
@@ -692,11 +722,11 @@ translated and passed through to that working plotting module.
 PDL::Graphics::Simple calls are dispatched in a two-step process. The
 main module curries the arguments, parsing them into a regularized
 form and carrying out DWIM optimizations. The regularized arguments
-are passed to subclasses that translate them into the APIs of their
-respective plot engines.  The subclasses are very simple and implement
+are passed to implementation classes that translate them into the APIs of their
+respective plot engines.  The classes are very simple and implement
 only a few methods, outlined below.  They are intended only to be
 called by the PDL::Graphics::Simple driver, which limits the need for
-argument processing, currying, and parsing. The subclasses are thus
+argument processing, currying, and parsing. The classes are thus
 responsible only for converting the regularized parameters to plot
 calls in the form expected by their corresponding plot modules.
 
@@ -708,7 +738,7 @@ non-object-oriented interface easier to implement since the main
 interface modules are in one place and can access the global object
 easily.
 
-=head2 Interface subclass methods
+=head2 Interface class methods
 
 Each interface module supports the following methods:
 
@@ -775,9 +805,7 @@ The hash in the curve block contains the curve options for that
 particular curve.  They are all set to have reasonable default values.
 The values passed in are C<with> and C<key>.  If the C<legend>
 option is undefined, then the curve should not be placed into a plot
-legend (if present).  The C<with> option will be one of C<points>,
-C<lines>, C<bins>, C<errorbars>, C<limitbars>, C<circles>
-C<image>, or C<labels>.
+legend (if present).
 
 =head1 ENVIRONMENT
 

lib/PDL/Graphics/Simple.pm

@@ -689,6 +689,20 @@ as they are interpreted as 8 bits per plane colour values. E.g.:
   $image_data = rpic( 'my-image.png' )->mv(0,-1); # need RGB 3-dim last
   $w->image( $image_data );
 
+If you have a 2-D field of values that you would like to see with a heatmap:
+
+  use PDL::Graphics::ColorSpace;
+  sub as_heatmap {
+    my ($d) = @_;
+    my $max = $d->max;
+    die "as_heatmap: can't work if max == 0" if $max == 0;
+    $d /= $max; # negative OK
+    my $hue   = (1 - $d)*240;
+    $d = cat($hue, pdl(1), pdl(1));
+    (hsv_to_rgb($d->mv(-1,0)) * 255)->byte->mv(0,-1);
+  }
+  $w->image( as_heatmap(rvals 300,300) );
+
 =item contours
 
 As of 1.012. Draws contours. Takes a 2-D array of values, as (width x