Create node fields with values that change every week but old values are being stored My company has about 15 oil tanks. Every week a guy checks the remaining oil in every tank and writes down the numbers. I want to create a visual representation of the remaining oil in every tank. I have a drupal installation already setup in my server and I thought that I could incorporate a solution in the existing site. What I have done so far:


I have created a new content type called tanks.
Created the various fields (like tank_id, volume, status etc) that will hold information about the tank.
I also have a field that is called latest_amount where I want to store the most recent measurement of oil in the tank.
I have also written some basic php code that does a simple division (latest amount/volume), gets the percentage of the tank that is full and I'm thinking about drawing with html a basic shape that will be green if it's above 75%, yellow if it's 25%-75% and red if it's below 25% (maybe there is a module that can help me with this drawing). 


My problem is that I want to keep past measurements so that I can create a graph with these percentages over the last 3 months. Is there any way for me to keep these stats along with the date that the specific field value has been added? Also, is there a way to restrict node editing to only adding a new value in the latest_amount field every week (i.e. not being able to alter any other information about the tank)?
Check the module field_collection


  Provides a field-collection field, to which any number of fields can
  be attached.



add field collection to your tanks content type
in collection add fields for stats like date, amount etc
after that you can store latest amount and display them in views, charts etc