Explanation of global variables

3d-variables:
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x/y/zorg: user-coordinates of origin
x/y/zscale: scaling factors (millimeters per user unit
x/y/zeye: eye-position in mm
x/y/zfocus: focus position in mm
x/y/zdis: x/y/zeye-x/y/zfocus
x/y/zpen3: current pen-position in mm
d3: distance between eye and focus in mm
d3d3: d3**2
d2: length of d3, projected on the xy-plane

bold variables: 
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pndia: boldness of lines and pen diameter set by user

angle variables: 
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x/yangle: angle of x- and y-axis (default 0 and 90 degrees)
x/ysin,x/ycos: sinus and cosinus of x/yangle

axis variables: 
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xin: 1, scale marks plotted above x-axis; -1, below x-axis
yin: 1, scale marks plotted right from y-axis; -1 left from y-axis
x/ysquar: if true, numbers plotted square to axis, else in its direction
x/ygrid: grid to be drawn square to x- or y-axis
border: graph to be closed to a rectangle
numlen: maximum length of numbers plotted at axes without
        scaling 10-power (default 6)
x/ymark: distance between scale marks in user units;
             0, automatic determination
x/yskip: plot of one number per x- or yskip scale marks

origin variables: 
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x/yoff: coordinates of relative origin
x/yorig: coordinates of origin (in user units)
x/ycross: if cross, coordinates of origin else meaningless
cross: axes intersect at (xcross,ycross)

pen variables: 
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x/ypen: absolute coordinates of new pen position in plits
x/yvirt: absolute coordinates of virtual pen position in plits
x/ypre: absolute coordinates of previous pen position
lastic: pen up (3) or pen down (2)
xyflag: true: next p0(z) or p0m(z) will be in x direction, else y

scale variables:
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sx/y: scaling factors (number of plits per user unit)
plmm: if true, user units are system units.
x/ylog: 0, non-logarithmic axis (default);
        1, logarithmic axis; -1, candidate logarithmic axis

dimensions of plot (window):
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x/ysize: (maximum) 'page' dimensions in plits
x/yextlo,x/yexthi: outer limits so far reached on page;
                        used in /plfram/ and /p0odir/.
x/ywinlo, x/ywinhi: coordinates of window borders;
x/ybase: lower left corner of area reserved by plorig

