Since there are no input graphs, I will be using the following three simulated graphs

In order to incorporate a letter indicator for each graph I will be using Epilog
along with Inset
with scaled coordinates (see Scaled
).
Evaluating
Grid[
Through[
{{First[#], SpanFromLeft} &, Rest}[
MapThread[plot, Transpose@args]
]
]
]
produces

code
(* functions used for display purposes *)
funcs = {Cos[#] - Sin[3 # - 2] &,
-12.1 #^3 + 10 #^2 - 3.02 # + 12 &,
2.1 #^4 - 3.2 #^3 + #^2 - 3.02 # + 12 &};
(* ranges used for plotting functions *)
rng = {{-2 Pi, 2 Pi}, {-4.1, 3.2}, {-2.7, 3.1}};
(* size of each plot - relative size differences are
important in order to produce the effect present in the question *)
sz = {Large, Medium, Medium};
(* the symbols used to index the different plots *)
sym = {A, B, C};
(* combination of expressions to be used as input - see below *)
args = Transpose[{funcs, rng, sz, sym}];
plot[f_, rng_, sz_, sym_] := Module[{x},
Plot[
f[x],
Flatten[{x, rng}],
Frame -> True,
Axes -> False,
ImageSize -> sz,
Epilog -> {Inset[Style[sym, Bold, 22], Scaled[{0.9, 0.85}]]}
]
]
Edit
Alternatively, the rhs of the Epilog
option can be modified to Inset[Style[sym, Bold, 22], Offset[{-30, -30}, Scaled[{1, 1}]]]
as suggested in the comments by @Carl Woll. Using Offset
with Scaled
provides better control over the position of the label.
Legended[BarChart[RandomInteger[10, 10], ImageMargins -> {{5, 0}, {0, 0}}, PlotRange -> All, Frame -> True, AspectRatio -> 1/2, ImageSize -> 600], Placed[Style["A", 24, "Panel", Background -> Transparent], {{Right, Top}}]]
? $\endgroup${Scaled[.95], Scaled[.9]}
for the top plot and{Scaled[.9], Scaled[.9]}
for the bottom ones in place of{{Right, Top}}
. $\endgroup$