1. 圖形文字
Text Labels: Use parse = T inside geom_text or annotate.

2. 座標軸標籤
Axis Labels: Use expression(alpha) to get greek alpha.

3. Legend
Legend Labels: Use bquote(alpha == .(value)) in legend label.

4. Fecet圖形
Facet 圖形比較特殊，要將 labeller = label_parsed
Facet Labels: Use labeller = label_parsed inside facet.

來源