KAlgebra is a simple computer algebra system and graphing calculator for Linux. The 2D and 3D plotting is excellent and a replacement to using an actual graphing calculator. I find it useful to plot functions instead of using SageMath, which can be an overkill for just plotting.
 It can be installed easily from Ubuntu repositories:
$ sudo apt install kalgebra
 You can see 4 tabs in KAlgebra: calculator, 2D graph, 3D graph and dictionary.

Calculator: Type any function here to simplify or solve it. For functions with one variable, KAlgebra gives a Plot 2D link after you press Enter. Clicking it takes you to the 2D Graph tab which renders the 2D plot of the function. Similarly, if you enter a function with two variables, it gives a Plot 3D link that takes you to rendering of its 3D plot in the 3D Graph tab.

2D Graph: Type a function of one variable here and KAlgebra plots its 2D graph. It checks if the expression is correct as you type and indicates error as tooltip. If expression is correct, then it shows the graph preview right below. If you press Enter after the expression, it is plotted in the main 2D plot window. More functions can be added by clicking the Add tab in the bottomright corner. As you move the mouse inside the 2D plot window, KAlgebra shows the X and Y coordinate values in the bottomleft corner. It also shows the tangent to the curve as you move your mouse along the curve.

3D Graph: Type a function of two variables here and KAlgebra plots its 3D graph. I found that KAlgebra plots 3D graph only for the first function. Any subsequent functions you type will not be plotted. This glaring bug is documented here.

Dictionary: This lists and describes the 64 functions that can be used in KAlgebra. It shows the function name, its input parameters and description.
Tried with: KAlgebra 15.12.3 and Ubuntu 16.04