[Rd] axis() ignores supplied value of argument mgp[3]?

J. R. M. Hosking jh910 at juno.com
Thu Aug 7 02:26:59 CEST 2008

Prof Brian Ripley wrote:
> On Sun, 3 Aug 2008, J. R. M. Hosking wrote:
>> Prof Brian Ripley wrote:
>>> What is true is that the line is set before the inline setting of mgp 
>>> is processed: it all depends on what 'supplied' means, and you 
>>> supplied two values.
>>> This is changed in R-patched now.
>> Thank you, that works.  Here is a perhaps related issue:
>> R Help on 'axis' for argument 'line' says "The relative placing of
>> tick marks and tick labels is unchanged".  But this appears not to be
>> the case when graphics parameter mgp[3] is nonzero:
>>  plot(1:4, yaxt="n")
>>  par(mgp=c(3,2,1))
>>  axis(2, 1:2)
>>  axis(2, 3:4, line=1)
>> which one (or, at least, I) would expect to place all the tick labels
>> the same distance into the margin, does not in fact do so.
> I don't think that is what it says. Setting 'line' or 'pos' temporarily 
> sets mgp[3] to zero (which is what the help implies in parts you did not 
> quote).

I'm afraid I read the help closely several times and did not find that

> Otherwise 'line' would just duplicate mgp[3].

Which is still useful functionality --- easier to supply a single value
than to change one element of a 3-element vector.

>> My interpretation is that axis() is taking mgp[2] rather than the
>> correct mgp[2]-mgp[3] as the relative position of the tick mark with
> Why is it correct when you have been told 'mgp[3]' was overridden?

That was my best attempt to interpret the phrase "relative placing of
... tick labels", given that no graphics parameter is explicitly
documented as having that meaning.

> We could explicitly say it is treated as zero for ultimate clarity.
>> respect to the axis line.

To summarize (if I now understand things correctly): when the position
of the axis line is determined by the value of mgp[3], then mgp[2] is
the absolute position (number of lines into the margin) of the tick
labels; but if the position of the axis line is determined by the 'line'
or 'pos' arguments of axis(), then mgp[2] is the relative position of
the tick labels (number of lines between the tick labels and the axis
line).  If this is the intended behaviour, then certainly I think it
would be helpful if it were explicitly documented.

J. R. M. Hosking

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