Crosspost of my answer in the GitHub issue (https://github.com/JuliaPlots/Plots.jl/issues/3013):
You can provide the ticks and labels manually by passing a tuple of tick positions (vector of reals) and tick labels (vector of strings) to the ticks
attribute:
plot(
y_grid,
x_grid,
color_grid,
yflip = false,
ticks = (range(1, 100, length = 5), ticklabels),
)