Trouble shooting

Here we collect a couple of common problems and their remedies.

<E> ... ambiguous command/parameter
<E> invalid parameter

   Happens in cases like:

   GWRITE a/b

   i.e. when a text string contains slashes (/).
   Because a slash is a separator for keyword command parameters
   it cannot appear in unprotected text strings.

   Cure: Enclose the text in quotes:

   GWRITE "a/b"

<E> can't set display scale

   Occurs if you try to do logarithmic plots and the data range extends into
   the negative region. Or if the upper limit is less or equal to the lower limit.

   Check the XMIN, XMAX, YMIN, YMAX display parameters.

<E> can't open display

   Happens on X11-based systems when an X11-Window cannot be opened on the selected X-server
   or if the environment variable $DISPLAY has not been set yet.

   Set the $DISPLAY variable correctly, e.g.:

   export DISPLAY=xserver:0

   and/or check the X-server permission.

I get only incomplete PostScript pictures ... 

   Use the command GPCLOSE to terminate the pictures before printing.

<E> gdh2blk.c: number of 2D data inconsistent
<E> gdread.c: number of 2D data inconsistent

    Line 3
    H2: X=-.2 TO .45 BY .05; Y=-.24 TO .12 BY .04;

   indicates a mismatch between the number of x- and/or y-values determined
   from the H2:-header and the following data.
   Probably caused by round-off errors during loop execution.
   Use integer expressions to specify the x- and y-loops and scale them afterwards:

   H2: (x=-4 to 9)/20 (y=-6 to 3)/25 z