Can you clarify what you data format is? Do you mean to say that you have 355 data files of raw data? Do the files have headers? What are the dimensions of thes files? Also How do fluorescence intensity and number of cells relate to your data (i.e. is number of cels a column or row value)? What is the file extension of your data files? Is it .xls, .csv...? It would be really difficult to help you get started with writing a code without knowing things like the dimensions of your data, how it is organized, etc.
Also, a histogram essentially displays the number of occurences number ranges (bins) in a data set so you will always have some number ranges (bins) on the x-axis and the count (number of times that the data fell into each bin in the data set). So if you are looking to plot your x vs. y or something like that, a histogram is not the tool to be using.
