= Solution
The <photon propagator> gives approximately $d\sigma_{\mathrm{NC}}/dQ^2\propto1/(Q^2)^2$, whereas a massive <W boson> gives $d\sigma_{\mathrm{CC}}/dQ^2\propto1/(Q^2+M_W^2)^2$. Up to different <coupling constant (physics)> values, <parton distribution functions> and angular factors,
$$
\frac{\sigma_{\mathrm{CC}}}{\sigma_{\mathrm{NC}}}\simeq C(Q^2)\left(\frac{Q^2}{Q^2+M_W^2}\right)^2.
$$
Consequently the low-$Q^2$ <charged current> suppression disappears as $Q^2$ approaches $M_W^2$. The source graph shows comparable <cross sections> at $Q^2$ of order $10^4\ \mathrm{GeV}^2$, so the relevant <mass> scale is of order $10^2\ \mathrm{GeV}$.
A more quantitative, still rough reading uses the $e^-p$ ratio $R_1\simeq0.006$ near $q_1=300\ \mathrm{GeV}^2$ and $R_2\simeq0.17$ near $q_2=2000\ \mathrm{GeV}^2$, where $q_i$ denotes $Q^2$, not $Q$. If $C$ varies slowly over this interval, set $\kappa=\sqrt{R_2/R_1}\,q_1/q_2$. Then
$$
\kappa=\frac{q_1+M_W^2}{q_2+M_W^2},\qquad \boxed{M_W=\sqrt{\frac{\kappa q_2-q_1}{1-\kappa}}\simeq80\ \mathrm{GeV}}.
$$
The reading is compatible with roughly $70$–$100\ \mathrm{GeV}$; it is not a precision <mass> determination. In particular, <Z boson> exchange contributes to the <neutral current> at high $Q^2$, and the <parton distribution functions> do not cancel exactly. Equality of two inclusive <cross sections> alone does not define $M_W$.
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