Why "" is considered as a number

Hi everyone,

I have an array of x and y data but I want to plot only a few data selected by me.
For this reason I have created something like this: (have a look at the attached .jpg)

xdata ydata selection selected x data selected y data
A B C D E
1 1.204234 49.359627 x IF(C1="x";A1;"") IF(C1="x";B1;"")
2 1.202642 49.299625 x IF(C2="x";A2;"") IF(C2="x";B2;"")
3 1.201362 49.239628 IF(C3="x";A3;"") IF(C3="x";B3;"")
4 1.199451 49.179626 x IF(C4="x";A4;"") IF(C4="x";B4;"")

But this gave me problems since "" is recognized as a number (or something else) and when I plot D and E columns the plot is wrong.
If I substitute the formulas with "0" instead of "" I get a right plot but since I have to interpolate the selected data with a linear trend on the plot, the unselected null data would cause me a wrong interpolation.
Why excel considers "" as a number (or something else) and what I can use instead of it?

Thank you very much for your help

Federicohelp.jpg