You almost got it. Maybe you just needed to read the manual some more and understand what is x and what is y.
p=[101.3 106.6 109.4 114.7 116.9 118.2 123.7];
v=[1.69 3.67 4.37 5.45 5.84 6.05 6.90];
plot(p, v, 'b.-', 'LineWidth', 2, 'MarkerSize', 20);
grid on;
xlabel('Pressure');
ylabel('Velocity');
% 4 ways for interpolation
xq = linspace(min(p), max(p), 500);
lin=interp1(p, v, xq,'linear') ;
m=interp1(p, v, xq,'makima');
sp=interp1(p, v, xq,'spline') ;
n=interp1(p, v, xq,'nearest');
% Find where speed is closest to 5
[value, index] = min(abs(lin - 5)) % Value is how far away from 5 the data point is.
x5 = xq(index)
y5 = lin(index)
hold on;
yline(5, 'Color', 'r') % Horizontal red line at 5.
xline(x5, 'Color', 'r') % Horizontal red line at 5.
% Plot a red asterisk at the point closest to y=5
plot(x5, y5, 'r*', 'MarkerSize', 30, 'LineWidth', 2)
Go ahead and add the rest, like legend, title, etc.
