You can add a second curve that is outside the plot range and denote the symbold that you would like to see in your legend to this second curve. Then you make a legend and assign the text formally to the second curve.
Example:
figure
x_data = 1:10;
y_data = 700*rand(1,10);
plt1=plot(x_data,y_data,'r-d')
hold on
plt2=plot(-100,-100,'*')
set(gca,'Xlim',[1,10],'Ylim',[0,800])
legend(plt2,{'data1'})
