Re: GridLines in LogLogPlot

• To: mathgroup at christensen.cybernetics.net
• Subject: [mg840] Re: GridLines in LogLogPlot
• From: cal at star.stanford.edu (Calvin Teague)
• Date: Tue, 25 Apr 1995 02:21:26 -0400
• Organization: Stanford University

```In article <3n4mqg\$q7o at news0.cybernetics.net>, sergio at shark.inst.bnl.gov
(Sergio Rescia) wrote:

> Dear Mathgrouper,
>
> Please take a look at the attached graphics file (It is a LogLogPlot: it
needs
> Graphics`Graphics` to display).  [omitted]
>
> The grid lines at the bottom, horizontally are wrong.
>
> There are correctly grid lines at y=2. 10^-6 and 5. 10^-6.
> I would expect two more grid lines at y=3. 10^-6 and y=4. 10^-6. Instead
there is only a single
> grid lines which turns out to be at y=3.5 10^-6.
>
> This is mathematically correct, of course, but nevertheless it is very
confusing
> when showing this kind of plots to an engineering audience.
>
> Is there a fix to allow for grid lines in a 2-3-4-5...or possibly a
2-5-10 sequence in a LogLogPlot?

I noticed similar behavior with LogLogPlot.  The problem appears to
involve rounding in a floating-to-integer conversion.  After much tracing,
I was able to correct it by adding a very small bias
(10^-(\$MachinePrecision-1)) to the argument of Floor[] in the function
MinorAux1[] in my copy of  Graphics.m  as follows:

Change

MinorAux1[x_] :=
Module[{n=Floor[ x/10^Floor[ Log[10., x]]]},
If[ n < 1, {1,1}, {x/n, n}]
]

to

MinorAux1[x_] :=
Module[{n=Floor[ x/10^Floor[ Log[10., x]] +
10^-(\$MachinePrecision-1) ]},
If[ n < 1, {1,1}, {x/n, n}]
]

I can't guarantee that this will work in all cases, but it did work for
the cases that I tried (involving similar small values for the variables
plotted).

Calvin Teague, cal at star.stanford.edu

```

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