The problem of dataset imbalance has raised a wide concern in many machine learning areas, but not in non-intrusive load monitoring, or load disaggregation. In this study, a pictorial evaluation method is proposed to representation the imbalance class distribution in datasets. We colored a Karnaugh maps according to the quantities of different variables combination to offer a visual impact to the whole dataset. After utilizing this method on a public dataset and its testing result, a clear imbalanced abundance in the dataset and an exciting performance have been found. A preliminary Python package to realize this mapping method has been uploaded on GitHuba.