Re: plot using cbticlabels: an unfinished and undocumented feature

[sfeam <sf...@us...>]

On Saturday, 28 July 2018 22:35:00 Juhász Péter wrote:
> Hello,
> 
> I wanted to create a graph where tics on the colorbox are dynamically
> generated from data. I vaguely recalled that there was a feature for
> that, and it seemed logical that if there is {x|y}[2]ticlabels() to
> place tics on the main axes, there should be a cbticlabels to do the
> same for the colorbox.
> 
> It seems that this is valid syntax, as the parser accepts it, and I've
> found the code responsible for it in the source, but it appears to be
> undocumented, and there are no demos for it either.
> 
> It also appears to be either half-finished or buggy. 

All true.  "cbticlabels" was added for completeness when the other
ticlabels variants were introduced, but it was never fully thought
out or documented. 

[snip]
> See the attached data file and gnuplot script for a self-contained
> example.
> 
> The fix, at least from the user's perspective, seems simple: make
> cbticlabels use the same column that is used for the color data itself.
> From the developer's perspective, I realize that this might be hard,
> because we don't yet know which column to use at the time of parsing
> the using spec, because an unknown number of `xxx variable` statements
> may come later.
> 
> Or - `help pointsize variable` states that variable color is alwasy
> taken from the last additional column. That we do know.

Yes.  I think that would be sufficient to cover all your test cases
in 2D plots. Like this:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
diff --git a/src/datafile.c b/src/datafile.c
index 8baf1d5..208ca3a 100644
--- a/src/datafile.c
+++ b/src/datafile.c
@@ -2059,7 +2059,7 @@ df_readascii(double v[], int max)
                            break;
                        case CT_CBTICLABEL:
                            axis = COLOR_AXIS;
-                       axcol = 3;
+                     axcol = df_no_use_specs - 1;
                            break;
                    }
                    /* Trap special case of only a single 'using' column */
@@ -5315,7 +5315,7 @@ df_readbinary(double v[], int max)
                        break;
                    case CT_CBTICLABEL:
                        axis = COLOR_AXIS;
-                   axcol = 2;
+                 axcol = df_no_use_specs - 1;
                        break;
                }
                if (a.type == STRING) {
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

However that fix doesn't work so well for 3D plots, where there is always
a z coordinate and that is usually (but not always) where the color comes
from.  The code in datafile.c is shared by both 2D and 3D plots so it 
would be tricky to make it work for both.

	Ethan


> Best regards,
> Peter Juhasz