Gdtyjr

The user marmot has posted a really nice approach of adding legend items into one single box below a multigraph diagram by placing them inside a node.

Based on this initial situation I have tried to separate the legend items into three rows above each other, but the result is missleading.

Minimum Working Example (MWE):

documentclass[border=5mm]standalone
usepackagepgfplots
usetikzlibrarypositioning
pgfplotssetcompat=newest, scale only axis
begindocument

begintikzpicture
beginscope[local bounding box=plots]
 beginaxis[
 axis y line* = left,
 legend to name = legA,
 legend style = draw=none,
 legend columns = 3,
 /tikz/every even column/.append style = column sep=0.5cm,
 /tikz/every odd column/.append style = column sep=0.15cm,
 ,
 ]%
 addplot coordinates (0,0) (1,1);
 addplot coordinates (1,0) (0,1);
 addplot coordinates (0,1) (1,1);
 legendGraph 1, Graph 2, Graph 3;
 endaxis%
 %
 beginaxis[
 axis x line = none,
 axis y line* = right,
 legend to name = legB,
 legend style = draw=none,
 legend columns = 3,
 /tikz/every even column/.append style = column sep=0.5cm,
 /tikz/every odd column/.append style = column sep=0.15cm,
 ,
 ]%
 addplot coordinates (0,1) (1,0);
 addplot coordinates (0,0.5) (1,0.5);
 addplot coordinates (0,0.15) (1,0.15);
 legendGraphinggraph 4, Graphinggraph 5, Graphinggraph 6;
 endaxis%
 %
 beginaxis[
 axis x line = none,
 axis y line* = right,
 legend to name = legC,
 legend style = draw=none,
 legend columns = 3,
 /tikz/every even column/.append style = column sep=0.5cm,
 /tikz/every odd column/.append style = column sep=0.15cm,
 ,
 ]%
 pgfplotsset%
 %
 every outer y axis line/.style = xshift=2cm,
 every tick/.style = xshift=2cm,
 every y tick label/.style = xshift=2cm,
 %
 addplot coordinates (0.5,0) (0.5,1);
 addplot coordinates (0.6,0) (0.6,1);
 addplot coordinates (0.7,0) (0.7,1);
 legendGraphgraph 7, Graphgraph 8, Graphgraph 9;
 endaxis%
endscope
node[below=3.5em of plots.south] (legA) reflegA;
node[below=-0.5em of legA] (legB) reflegB;
node[below=-0.5em of legB] (legC) reflegC;
draw (legA.north west) rectangle (legC.south east);
endtikzpicture
enddocument

Screenshot of the result:

Description of the issue:

As you can see,

• the border does not fit with the outer dimension of the legend items,


• the legend items are not precisely aligned below each other.

So my questions are: How to solve this and make it look nice?

I will try to answer just your WHY

before @marmot awakes and sees me messing about with TikZ / pgfplots answers.

His answer was provided by the first poster (thus not strctly marmots choice) it depended on an automatic box that adjusts width so unless the text in each column is equal the legend will be unequal.

You could cheat by adding spaces to match the longest entry as I have done here

to save space I will only add the changed lines

 legendGrumpy children whine ~ ~ ~1, Graph with fixed width 2, Graph with fixed width 3;
 legendGrapes make good wine ~ ~ ~4, Graph with fixed width 5, Graph with fixed width 6;
 legendGraph with fixed width~ ~ ~7, Graph with fixed width 8, Graph~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~9;

This how to "fix" it is far from ideal, and a different answer is needed for how to replace it. so lets see if a question flag change will get you a real answer