The Rugrats really were a figment of Angelica's imagination.
Chuckie died a long time ago along with this mother; that's why Chaz is a nervous wreck all the time.
Tommy was a stillborn; that's why Stu is constantly in the basement making toys for the son who never had a chance to live.
The DeVilles had an abortion. Angelica couldn't figure whether it would be a boy or a girl, thus creating the twins.
As for "All Grown Up," Angelica was a bipolar schizophrenic, who, as a teenager, became addicted to various narcotics, bringing her back to her childhood, thus creating a world in her mind that she obsessed over. Because of the time lapse between the present and the last time she interacted with her imaginary world, she made them older. Angelica was constantally taking hits of acid so she would never have to live without her creations. To her, her creations were her only company in a judgemental world.
Angelica's mom actually died of a heroin overdose. Angelica was schizophrenic and bipolar because she was a crack baby. Additionally, Drew, in his depression, married a gold digging whore that Angelica idolized because she fooled herself into thinking it was her real mom. However, she always had a concept of her mom, Cynthia. She used a barbie doll to mirror her birth mother's image--wearing an unwashed orange dress and jacked up hair--which is why she was so attached to it. Later in life she followed in her mother's footsteps, dying of overdose at age 13 when All Grown Up! was "cancelled."
The only rugrat not to be fictional however, was unborn Tommy's brother, Dil. However, Angelica didn't know the difference between Dil and her creations. Dil didn't follow her commands and after endless crying and a refusal to disappear like the others did when Angelica was angry with them, so she hit him. After she hit him, he screamed a screeching tune, and Stu ran in and pulled his neice off of his only child, but it was too late. Dil had a brain hemorrage, which resulted in a deformation. As he grew up, his damage only became more evident, and by the time he was 9 in All Grown Up! he lived as an outcast, being ridiculed for his weirdness and retardation. The immense guilt over this is what caused Angelica to start using drugs, and to un-create the rugrats briefly, until her experience with hallucinogenics.
Chaz lost his mind after the death of his first wife and was in denial that she was ever a prostitute. On a trip to Paris to find love, Chaz fell in love with a hooker named Kira (He was originally going to marry a different hooker, but she just wanted him for his money). Kira once had a daughter named Kimi, but the baby was torn from her by law due to her cocaine addiction. (Angelica imagined Kimi from Kira's stories.) Upon return to America, Chaz and Kira married and she got her greencard. It was a surprisingly happy and romantic story. Kira continually stuggled with addiction, but was relatively happy with her new life with Chaz.
Suzie was Angelica's only friend, who entertained the thought of Angelica's creations because they seemed to make her happy. She later became a phycologist and teamed up with Nickelodeon to make the Rugrats. When Angelica died of the overdose, Suzie helped arrange her funeral. Because of her addictions and her mental state, Angelica was expelled from society, which lead to a break with reality and her eventual death. She spent the last days of her life in the back of the school cafeteria, imagining friends around her and playing with the lives of her creations.
