I don't know so much GIS specific terminology, so I am sorry for using the wrong words. I'll try to make my point clear by first describing what I want to achieve and then what I tried.
[goal] I am working with tweets that I have stored in a csv file. Now I want to visualise them on a map. I want to sort them according to their date (or some other column in my file). So I want to have a way to display only tweets from a certain day or date or some other property. Also, because there are many tweets for each property, I want them to be clustered. So I can see from the point's colour how many are at the same spot instead of having to zoom in. I need this to be nicely displayed on a static picture.
[what I did] I loaded my .csv file and categorized it according to date. This worked, so now I have a layer with all tweets but I can switch on/off each date. They are not stored in a different layer, but as something else. I don't know how to call this. Then I tried the Heatmap Plugin, which I thought is what I need. But this only seems to work on normal layers. So I cannot visualise my categorized data in different parts, just the full layer. I also tried to categorise the heatmap somehow, but couldn't find out how.
I hope you can help me to achieve my goal. Of course I could create different .csv files for all the properties that I want to sort my data for, but that seems stupid. I think GIS should be able to do that easily.
By the way, I use QGIS 2.10.