Preheat oven to 350 F. Line a 9x5 inch loaf pan with parchment paper and spray with baking spray (or grease with butter).
In a small bowl, toss the plums in the 1 tablespoon of flour to coat; set aside.
In a bowl, whisk together flour, baking soda, and salt.
In a mixing bowl with electric mixer (or in another large bowl, by hand), beat margarine (or oil or butter), sugar and vanilla extract medium-high speed until well blended.
Add eggs, one at a time, beating well after each addition.
Add in the grated lemon zest. Mix in the yogurt and combine.
Beating at low speed, add the flour mixture to sugar mixture; beat just until combined.
If you find the batter too thick, add a 1-2 tablespoons of milk (or for the dairy free: use unsweetened almond or whatever your favorite nut milk is). Start with 1 tablespoon and see how the texture is. It is a thicker batter.
Gently fold in the plums until combined.
Spoon the batter into the prepared pan.
Sprinkle the cinnamon-sugar topping on top of the batter.
Bake at 350° for 30 minutes. Lower the heat to 325 and bake for about another 25-30 minutes, or until a wooden pick inserted in center comes out with moist crumbs clinging.
Cool 10 minutes in pan on a wire rack. Remove plum bread from pan. Slice and enjoy!