Formulas to plot Frequency Impulse Response Coefficients
Assuming you have a set of (windowed) filter coefficients $h[n]$, $n=0,1,\ldots,N-1$, and you want to evaluate the corresponding frequency response at $ M\ge N$ equidistant normalized frequency points

$$f_k=\frac{k}{2M},\quad k=0,1,\ldots,M-1\tag{1}$$ 

then you need to evaluate the following formula, which is the discrete Fourier transform (DFT):

$$H[k]=\sum_{n=0}^{N-1}h[n]e^{-j\pi nk /M},\quad k=0,1,\ldots, M-1\tag{2}$$

The actual frequencies in Hz are given by $f_k\cdot f_s$, where $f_s$ is the sampling frequency.

Equation (2) is exactly what you get with the Matlab command

[H,w] = freqz(h,1,M)

where $f_k=$ w(k+1)/(2*pi) (because Matlab indices start at $1$, not at $0$).

If you want to evaluate the frequency response at arbitrary, not necessarily equidistant normalized frequencies $f_k$, you need to evaluate

$$H[f_k]=\sum_{n=0}^{N-1}h[n]e^{-j2\pi nf_k},\quad 0\le f_k\le 0.5\tag{3}$$

which for the choice of $f_k$ according to (1) of course reduces to Eq. (2).