I have a text file having the numbers(of float type) which represents time in seconds. I wish to represent the number of occurances every 15 minutes. The sample of my file is: 

```
0.128766
2.888977
25.087900
102.787657
400.654768
879.090874
903.786754
1367.098789
1456.678567
1786.564569
1909.567567

```

for first 900 seconds(15 minutes), there are 6 occurances. I want to plot that point on y axis first. Then from 900-1800(next 15 minutes), there are 4 occurances. So, i want to plot 4 on my y-axis next. This should go on... 

I know the basic plot() function, but i don't know how to plot every 15 minutes. If there is a link present, please guide me to that link.