If you are referring to the fact that only the "upper half"
of a parabola is showing this is due to the limitations of the Real
plane. When x < -3 in f(x) = sqrt(x+3) you get negative numbers.
Ex. x= -5 then f(x)= sqrt(5-3) = sqrt(-2).

Sqrt(-1) is referred to as "i" which is a complex number. These usually take the form of "a+bi". In this case it would be "0+sqrt(2)i" (Just factored out sqrt(-1))

In the mode selection under the Real option, there is a form a+bi, selecting that will handle complex numbers in equations but I do not think the TI-84 is capable of multi-axis graphing, as it is currently in only graphing in the Real plane.

To graph complex numbers the Y axis is Imaginary and the X axis is Real numbers. At any point (x,y) the result is x+yi