Update docs

docs/formulas.md

@@ -5,12 +5,17 @@ Formulas are composed of macros on both sides, which are connected by `'~'`. It
 However, we don't support operators like it from `R`. So we only have `Y ~ X`, but not `Y ~ X1 + X2`. In order to implement this, we have to define a macro to add `X1` and `X2` up.
 
 We have three types of macros:
+
 1. `continuous`: extracts continues values from a variant, such as allele frequency, depth, etc.
 2. `categorical`: extracts categorical values from a variant, such as variant type, genotype, etc.
-3. `aggregation`: aggregates some values from a group of variants, such as mean of allele frequency from chromosome 1
+3. `aggregation`: aggregates some values from a group of variants, such as mean of allele frequency from chromosome 1. You can also add a filter to aggregation by `COUNT(1, filter=AAF[0.05, 0.95], group=CHROM)`
 
 See section `Macros` for details.
 
+!!! Tip
+
+    If you have both aggregations for Y and X, you can omit the group for either. The one without group will automatically use the group that is specified to the other one.
+
 ## Predefined figure types for different combination of `Y` and `X`
 
 |Y|X|default figure type|other available figure types|