Problem with 2-axis diagram

Hello there,

I have to creat a diagramm which displays on the left y-axis concentrations of nutrients and on the right y-axis the watertemperatures. on the horizontal x-axis at the bottom I want to show the time with specific dates.
The data of the temepertaure was measured every 15minutes over 6 months. The nutrient concentrations were measured only twice or three times in the last 3 months (so I already put the value "0" in all other data points which were measured every 15minutes).
SO now when I draw the diagram, only the value of the axis which I add first will be shown correctly. So when I first add all temperature data, the correct curve will be displayed but the 4 various curves for the 4 nutrient types which shall lay on the other axis, will be displayed only in the zero line (as if the values would be 0 for all of them). But if I add first the values for the nutrients, they will all be displayed correctly, but once I add the data for the temperature, the temperature curve wont be displayed correctly.

Another addionally problem with teh temperaure is that even if all values are entered with comma (its a German excel 2003 I use)the daigram shows the curve only at 35123 (thirty three thousand...) instead of 35,123 (thirty five. But in my excel cells I have the data is written with comma but when I click on the cell, in the upper line the number will be displayed without a comma.

I need help with this please. I am very grateful for every single idea or comment for there I am completely stucked here and dont know how or what to change.

Cheers, Tania