Heat map or density map in R

I am new to R and trying to create a heat map or density map to show my results. I have x data for 20 different locations and for 20 years. I am trying to show differences in locations as well as in years in the same map. my data looks like this. 1975 1977 1979 1981 1983 1985 1987 1989 Dallas 25 28 27 29 31 33 35 37 Houston 33 38 43 48 53 58 63 68 Lubbock