@ClaudioFiandrino Just tried out your code. The reason colour wasn't working was because it needs to be highlighter/colour. I guess that the options to the \highlight command should automatically be in the /tikz/highlighter path.

@ClaudioFiandrino Thinking about the vertical highlight, I'm not really sure what to do with that. The reason for putting it in was to highlight columns of a matrix. But then I' not sure that what I've done is the best way to do that. I can't really imagine much use for a vertical highlighter that works in the way I've written it (unless working with a language that writes top-to-bottom). There's a box version of the highlighting code which just draws a box,

and I guess it could make sense to draw a line from start to finish, irrespective of where they are located, perhaps offset.

@AndrewStacey Yes, indeed was sufficient to say \[\highlight[highlighter/colour=green!50]{x+y}=100\] (why I didn't think about it?). What do you think about the commands \tikzmarkhighlightset and \tikzmarkfadeset? They might help users that sometimes forgot, as me, the right path.. for the keys. :)

For what concern the vertical highlight, well, I think is important, maybe not only for matrices, in which there are also other methods to highlight rows/columns or single elements, but also for fractions (indeed in my trial code the fraction is not perfectly highlighted). As far as I know, and this concept is very well explained in the guide I wrote for the GuIT, there are two methods: computing the box dimensions or let the user shift the markers.

Are the extent keys devoted for this purpose?

@ClaudioFiandrino I'm still pondering the vertical highlight. I'm trying to see if there is a case at all for the kind of "wrap around" facility that is in the horizontal highlight. For the column of a matrix then the "box" command might be better.

@ClaudioFiandrino I'm not sure that \tikzmarkhighlightset is any easier to remember than the path!

@AndrewStacey I tried to put myself in the position of explaining this to a complete newbie; then it's true that the command is just a shortcut, but it avoids saying that these keys are placed in a given path. And of course \tikzmarkhighlightset does not forbid to use anyway \highlight[highlighter/..=].. or \tikzset{/tikz/highlighter/.cd,....

@ClaudioFiandrino New version on launchpad. \usetikzmarklibrary is now the right one. I've changed the highlighting code considerably. The lengths are hopefully now much more intuitive.

There are initial offset, initial height, initial depth and final versions thereof. These define the coordinates of the _|- part of the highlighted box. From the marked position, _| is (-initial offset,-initial depth) and |- is (-initial offset,+initial height). There's a similar thing for the final position except that some +/- are reversed so that a positive length always enlargens the highligted region.

Margins are now left margin, right margin etc and again, positive means "bigger".

@ClaudioFiandrino Fair enough. But is \tikzmarkhighlightset the best name? How about \tikzmarkset{highlighter}{...} and \tikzmarkset{fader}{...}?

@ClaudioFiandrino There are some pictures in the tikzmark_test.tex which might make the lengths a little easier to understand. I haven't updated the documentation yet though.

@AndrewStacey That's even better! It is also much more simpler because the command is unique and holds for all extras (perhaps \tikzmarkset{listings}{...}).

@AndrewStacey I'm going to download it. :)