You are on the right track. You can use polyfit to fit a trend line to the data. The output of polyfit is a vector of coefficients corresponding to the polynomial you fit to the data. You can then use polyval for those coefficients to create the trend-line to add to the plot.
Your x-data for polyfit will be the dates, and the y-data will be the 91 values that you want to fit a straight line to.
Here's a quick example:
% Create and Plot Raw Data
x = 1:100;
y = 0.25*x + randn(1,100);
plot(x,y,'LineWidth',2)
% Fit line to data using polyfit
c = polyfit(x,y,1);
% Display evaluated equation y = m*x + b
disp(['Equation is y = ' num2str(c(1)) '*x + ' num2str(c(2))])
% Evaluate fit equation using polyval
y_est = polyval(c,x);
% Add trend line to plot
hold on
plot(x,y_est,'r--','LineWidth',2)
hold off
