how to set y-axis as log scale?

Method 1: Plot directly using semilogy

If you are generating a new 2D plot, use semilogy instead of the standard plot command:

x = 0:0.1:10;
y = exp(x);

semilogy(x, y, 'b-', 'LineWidth', 1.5);
xlabel('Linear X Axis');
ylabel('Logarithmic Y Axis');
grid on;

Method 2: Change an existing plot with YScale

If you already plotted your data using plot, scatter, or bar, modify the axes property directly without re-plotting:

% Dot notation (MATLAB R2014b and newer):
ax = gca;
ax.YScale = 'log';

% Classic syntax (works in all MATLAB versions):
set(gca, 'YScale', 'log');