We can create values of x and y and plot them in charts.
Add a label 'x' in cell A2. Create a sequence of values underneath the label from, say, -5 to +5.
In column B add a label 'y=x' and enter a formula to set each cell to the adjacent value in the A column (=A3...).
Select the two columns of data and create an X-Y (scatter) chart, choosing an option that will join the data markers.
In column C add another label 'y=x+3'. Enter a formula in cell C3 to perform the calculation. Select the x values in the A column, hold down the Control key and select the values in the C column. Create an X-Y (scatter) chart as before.
To add a 'Trend Line' right click any data marker and choose Add Trendline from the popup menu. Choose 'Linear' now and then click on the Options tab and tick the 'Display Equation on Chart' box.
Repeat the actions in the previous two stages for other equations such as 'y=x^2', 'y=x^2+x', 'y=x^2-x', 'y=2x^2+2x', 'y=x^3', 'y=x^4', and so on.
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