"plot" command for continous time signal in matlab?uses interpolation?
By default MATLAB uses linear interpolation when creating line plots, which means it simply draws a line from each point to each point, unless there are more points than pixels in which case each point or group of points within each pixel would represent each pixel shown. 

Compare the following to see this:

This plots a line from x,y=1,2 to x,y= 5,7

plot([1 5], [2 7])


The above is the same as the following where a line is explicitly declared:

plot([1 5], [2 7], '-')


While in comparison the following will plot a “o” at each sample:

plot([1 5], [2 7], 'o') 


Update: To interpolate more smoothly between samples such as polynomial interpolation, you would need to create the interpolated samples and then plot the higher sampled result. The simplest way to do this in MATLAB is to use the resample command where resample(f,M,N) will interpolate the waveform $f$ by $M/N$ samples. Another approach demonstrated below is interpolating using the FFT. Although DSP.SE is not a MATLAB coding site, the following does demonstrate well how to properly do a time domain interpolation using a zero-padded IFFT which is of general signal processing interest. Zero padding in one domain results in interpolation in the other domain, however the technique is to "zero-pad" from the center of the frequency domain waveform with index k from 0 to N-1, such that the high frequency component of the FFT is extended. 

N=6;
M=1000;
x= [0:N-1];
y = sin(2*pi*x/N);
fy = fft(y);
padfy = [fy(1:floor(N/2)) zeros(1, M-N) fy(floor(N/2+1:end))];
figure;
plot(N*[1:M]/M, M/N*real(ifft(padfy)));