Mfcttrf

tikz newbie here.

I want to make "template" drawing, which has part of it a line and then put different size labels on top of the line.

The label is some latex code (equation).

One of the problems is that, since I am using fixed coordinates each time, but the equation itself can be long or short, I need a way to make the latex font size automatically scale to fit the space.

Otherwise, each time I make new drawing, I have to do trial and error to change either the coordinates or manually change the font size. Here is MWE to better explain what I mean

documentclass[11pt]standalone
usepackagetikz

begindocument
begintikzpicture


coordinate[label=left:A] (A) at (0,0); 
coordinate[label=right:B] (B) at (3,0);

newcommandmyLabel$sin(x)+cos(y)=int_0^infty e^x y, dx$

draw (A) -- node[above] myLabel ++(B);

endtikzpicture
enddocument

Now, I can either try again with smaller font or change the coordinates to make the space larger so the new label fit.

I do not want to change the coordinates, since if these are part of larger diagram, it means I have to also change other coordinates as well and things will start to fall apart, because once I change one coordinate, I have to go update all the other coordinates to keep the shape the same.

So I try again with smaller font, like this

newcommandmyLabelfootnotesize$sin(x)+cos(y)=int_0^infty e^x y, dx$;

Ok, a little better

but still does not fit right. So I try now with

 newcommandmyLabeltiny$sin(x)+cos(y)=int_0^infty e^x y, dx$;

Ok a little better.

But this is all trial and error. I want a way to automate this by telling tikz to scale the text between 2 coordinates on a line to "fit".

What would be a good way to do this sort of thing? i.e I want labels to automatically "fit" in the space they are in without having to edit coordinates each time I change the label.

The general strategy is to measure the length of the path and then either measure the width of the node contents (first example) to compute the relevant scale factor, or to use adjustbox to scale the node to the length of the path (second example).

documentclass[11pt]standalone
usepackagetikz
usetikzlibrarycalc
begindocument
begintikzpicture


coordinate[label=left:A] (A) at (0,0); 
coordinate[label=right:B] (B) at (3,0);

newcommandmyLabel$sin(x)+cos(y)=int_0^infty e^x y, dx$
pgfmathsetmacromywidthwidth("myLabel")
draw let p1=($(B)-(A)$),n1=veclen(x1,y1) in (A) -- 
node[above,scale=n1/mywidth] myLabel ++(B);

endtikzpicture
enddocument

You can turn this into a style. This case can be adjusted to the actual use cases, which I do not know.

documentclass[tikz,border=3.14mm]standalone
usetikzlibrarycalc
usepackageadjustbox
begindocument
begintikzpicture[fitting label/.style args=between #1 and #2 with #3%
insert path=let p1=($(#2)-(#1)$),n1=veclen(x1,y1)
in (#1) -- (#2) node[midway]adjustboxwidth=n1#3
]


coordinate[label=left:A] (A) at (0,0); 
coordinate[label=right:B] (B) at (3,0);

newcommandmyLabel$displaystylesin(x)+cos(y)=intlimits_0^infty mathrme^x y,
mathrmdx$
draw[above,fitting label=between A and B with myLabel];
endtikzpicture
enddocument