How to fit a linear line on a semilog plot?

I'm struggling to fit a linear line on my log linear regression. Here is my code:

figure(2)
subplot(2,2,1)
[slope7, intercept7, MSE7, R27, S7, extra7] = logfit(totSPM,s2gbratio,'logx') % logfit is a function from the file exchange, uses semilogx.

fit7 = fitlm(log(totSPM),s2gbratio) % get x1/intercept/SE 
hold on

% confiedence intervals. Highest line. 
hx1 = (0.1546 + 0.036612); % get highest x1 value (x1 + SE)
hInt = (intercept7+0.077203); % get highest intercept value (intercept + SE)
x= linspace(0,10, 50); % Adapt n for resolution of graph 
y= hx1*x + hInt; % highest confidence interval I want to plot as linear
plot(x,y,'b--') % plot highest confidence interval

As its on a semilog plot the output is a curve.

Is there any way round this ?

I've attached an image of my output using this code (sorry bout the volume bar... oops)

Kind regards, Duncan