How could I find  lift coefficient slope?
Basic maths:

$\alpha_0$ = point where the line crosses the x-axis = -4

$C_L(\alpha)$ is linear, so take any two points, e.g. (-4,0) and (16,0.5)

$\Delta y$ = 0.5 - 0 = 0.5; $\Delta x = 16 -(-4) = 16 + 4 = 20$

$$C_L = \frac{0.5}{20} * (\alpha + 4) = 0.025 * (\alpha + 4)$$

Check:


$\alpha = -4: C_L = 0$
$\alpha = 16: C_L = 0.025 * (16 + 4) = 0.5$