From the graph image you linked, it appears that you are graphing time-series information.

Depending on your problem domain and end-user requirements, you may want to consider comprehensive and automatic tools for gathering, managing and displaying information that is typically time-series based.

The tools I'd recommend are based on rrdtool, which handles the database and graphing. Just this tool will probably meet requirements stated in your post. Web display and automation is achieved with MRTG as a starting point - when you are comfortable with that, upgrade to CACTI. These are typically used for network monitoring, but have applications in may other fields, such as temperature, and surf monitoring.

One thing to note is that these tools automatically manage data such that older data is displayed with less resolution. This helps keep the total database size constant. If this is acceptable, these tools will most likely meet your needs, and resolve issues you didn't know you had.