diff --git a/src/compile/format.ts b/src/compile/format.ts
index 0fd41358ee..53a727ded6 100644
--- a/src/compile/format.ts
+++ b/src/compile/format.ts
@@ -61,6 +61,7 @@ export function formatSignalRef({
   const field = fieldToFormat(fieldOrDatumDef, expr, normalizeStack);
   const type = channelDefType(fieldOrDatumDef);
 
+  // FIXME: this `config.customFormatTypes` clause should wrap all code relevant to formatCustomType
   if (format === undefined && formatType === undefined && config.customFormatTypes) {
     if (type === 'quantitative') {
       if (normalizeStack && config.normalizedNumberFormatType)
@@ -337,6 +338,7 @@ function wrapFormatExprToHandleInvalidValues({
   return mainFormatExpr;
 }
 
+// FIXME: we shouldn't really export this to be called directly
 export function binFormatExpression(
   startField: string,
   endField: string,
diff --git a/src/compile/mark/encode/tooltip.ts b/src/compile/mark/encode/tooltip.ts
index a3b5b68d39..90eeff6365 100644
--- a/src/compile/mark/encode/tooltip.ts
+++ b/src/compile/mark/encode/tooltip.ts
@@ -100,6 +100,7 @@ export function tooltipData(
 
     let value: string;
 
+    // FIXME: why doesn't this file just call formatSignalRef directly?
     if (isXorY(channel)) {
       const channel2 = channel === 'x' ? 'x2' : 'y2';
       const fieldDef2 = getFieldDef(encoding[channel2]);