Re: rgbplot composite fails with gnuplot

[Juan Pablo Carbajal]

On Tue, Dec 16, 2014 at 4:25 PM, Juan Pablo Carbajal
<address@hidden> wrote:
> On Tue, Dec 16, 2014 at 3:31 PM, Ben Abbott <address@hidden> wrote:
>>> On Dec 16, 2014, at 8:26 AM, Juan Pablo Carbajal <address@hidden> wrote:
>>>
>>> Tested in 4.1.0+
>>> repo id 19402:5cd83b466a3e
>>>
>>> octave-cli:1> rgbplot(jet(16),'composite')
>>>
>>> multiplot> plot "-" binary array=16x2 scan=yx origin=(1,1) dx=1 dy=0
>>> using 1 title "" with image; ;
>>>                                                                  ^
>>>           line 0: Sample period must be positive. Try `flip` for
>>> changing direction
>>>
>>>
>>> Can anybody reproduce?
>>>
>>> Do we report bugs that involve gnuplot?
>>>
>>
>> I get the same result using gnuplot 4.6 patchlevel 6.
>>
>> The error is indicating that dy must be greater than 0.  The m-code which 
>> produces this gnuplot command is in plot/utils/private/__go_draw_axes__.m at 
>> lines 554-547.
>>
>>         titlespec{data_idx} = "title \"\"";
>>         usingclause{data_idx} = sprintf ("binary array=%dx%d scan=yx 
>> origin=(%.15g,%.15g) dx=%.15g dy=%.15g using %s",
>>             x_dim, y_dim, x_origin, y_origin, dx, dy, format);
>>         withclause{data_idx} = sprintf ("with %s;", imagetype);
>>
>> The image intended looks a similar to a colorbar, so I expect the problem is 
>> on Octave's end.
>>
>> Ben
>
> Ben,
>
> Ok, thanks. I spotted the problem.
> It happens when when the data of the image has a single row.
> That is this fails as well
>
> image (1:8)
>
> The issue comes in lines 513 and 527-529 of __go_draw_axes__.m
> When dy=0 because the ydata of the image is [1 1]
>
> What are the "dx" and "dy" option for plot in gnuplot? I can't find a
> reference. Knowing that we jnow how to fix the issue.

Ok, found it.

The clause for gnuplot should not contain "dy=" when dy==0 . This
makes the plot appear but the axis look wrong.
So the sprintf command should be conditional on the value of dy.