Apply SortBy and then ordering elements

Question

I have the list:

a = {{1.0, 3.0}, {4.0, 3.0}, {1.0, 4.0}, {4.0, 1.5}, {1.2, 4.0}, {2., 
   8.}, {9.0, 9.0}};

When I apply SortBy[a,Last] I get the following:

{{4., 1.5}, {1., 3.}, {4., 3.}, {1., 4.}, {1.2, 4.}, {2., 8.}, {9., 
  9.}}

My question: For those with similar Last part, How can I order them from Largest to smallest, so the final output would look like:

{{4., 1.5}, {4., 3.}, {1., 3.}, {1.2, 4.}, {1., 4.}, {2., 8.}, {9., 
  9.}}

Thanks

Answer 1

You could use something like:

SortBy[a, {Last, Minus@*First}]

{{4., 1.5}, {4., 3.}, {1., 3.}, {1.2, 4.}, {1., 4.}, {2., 8.}, {9., 9.}}