Whyyaxis question? Because you wanted an answer!
Since plotyy is no longer recommended, here's a workaround with yyaxis. You can make it into your own %<UserProfile>%\Documents\MATLAB\ folder for future use.
close all; clear all; clc;
% Setup dummy data
x = 1:10;
y = x;
% Setup colors
line_color = [ 1 , 0 , 0 ]; % Red
alpha_value = 0.15;
grid_line_color = [ line_color , alpha_value ]; % Transparent red
% Create a figure and plot the dummy-data line
fig1 = figure(1);
ax1 = axes;
hold on;
yyaxis right;
line1 = plot( x , y , '-' , 'Color' , line_color );
myYGridHandles = plotRightYGrid( ax1 , grid_line_color );
function [ gridLineHandles ] = plotRightYGrid( axh , grid_line_color )
% Made into a function if you'd like to reuse in the future.
% The following is possibly redundant, but performed for sanity's sake
axes( axh );
yyaxis right;
% Acquire the x-axis limits to get the grid-lines' extents
xlim_right = xlim;
% Acquire the tick mark values; these can be default or specified by user
y_ticks_right = get( gca , 'YTick' );
% Plot the major Y-right grid lines
for( j = 2 : ( length( y_ticks_right ) - 1 ) )
gridLineHandles(j) = plot( [ xlim_right(1) , xlim_right(2) ] , ...
[ y_ticks_right(j) , y_ticks_right(j) ] , ...
'-' , 'Color' , grid_line_color );
end
end
