Re: key positioning problem

[Ethan A M. <merritt@u.washington.edu>]

On Sunday 22 February 2009, Allin Cottrell wrote:
> Problem: with both the png (libgd) and cairopng terminals,
> depending on the font selected the gnuplot "key" may overflow the
> plot data region (even if the key terms are in no way excessive,
> e.g. n <= 8 characters per term).

Gnuplot does not do any font handling on its own. It leaves this
to the individual external devices or libraries.  The corresponding
drivers are supposed to provide approximate information about the
average character width in the fields term->h_char, but the 
approximation can be pretty terrible depending on what font you pick.
And anyhow, at best it contains the estimated width of an "average"
character.  In your case the titles are in all caps, and these are
wider characters than average.

So if you think that the png terminals *always* underestimate the
font width, then you could multiply term->h_char by some constant
everywhere that it is set in the driver.  But I find it more likely
that some font widths will be underestimated and others will be
overestimated, and there is not much we can do about it.

In other words,  I don't think this is fixable. 
Not for png, and not for any other terminal unless it is using a
monospaced font and correctly reports the corresponding fixed width.

This is why there are fudge factors for the key layout, so that you
can tweak the layout if the automated estimation is failing.
In the example you give, it is sufficient to reserve a little extra width:

  set key width 2


> Example:
> set term pngcairo font "FreeSans,8" size 680,400
> set output 'test.png'
> set key left top
> plot x title "PRIME" w lines , \
>  2*x title "UNEMP" w lines
> 
> In the resulting plot, the 'U' of "UNEMP" is vertically bisected
> by the y axis.  The desired result is that all the key text
> appears within the data area (i.e. to the right of the y axis);
> and this is achieved if we change the first line of the plot file
> to (for example)
> 
> set term pngcairo font "Vera,8" size 680,400
> 
> I'd be happy to try to work up a patch for this, but I'm having
> difficulty figuring out where to start -- i.e., determining where
> the left limit of the key block is computed, given the selected
> font.
> 
> P.S., on further investigation this does not seems to be very
> terminal-specific: I'm getting the same issue with the pdf,
> pdfcairo, and 'post eps' terminal types.

-- 
Ethan A Merritt